Initial commit of OpenSPARC T2 design and verification files.

[OpenSPARC-T2-DV] / tools / perl-5.8.0 / lib / site_perl / 5.8.0 / sun4-solaris / Date / Calc.pod

=head1 NAME

Date::Calc - Gregorian calendar date calculations

=head1 MOTTO

Keep it small, fast and simple

=head1 PREFACE

This package consists of a C library and a Perl module (which uses
the C library, internally) for all kinds of date calculations based
on the Gregorian calendar (the one used in all western countries today),
thereby complying with all relevant norms and standards: S<ISO/R 2015-1971>,
S<DIN 1355> and, to some extent, S<ISO 8601> (where applicable).

(See also http://www.engelschall.com/u/sb/download/Date-Calc/DIN1355/
for a scan of part of the "S<DIN 1355>" document (in German)).

The module of course handles year numbers of 2000 and above correctly
("Year 2000" or "Y2K" compliance) -- actually all year numbers from 1
to the largest positive integer representable on your system (which
is at least 32767) can be dealt with.

This is not true, however, for the import/export functions in this
package which are an interface to the internal POSIX date and time
functions of your system, which can only cover dates in the following
ranges:

 01-Jan-1970 00:00:00 GMT .. 19-Jan-2038 03:14:07 GMT [Unix etc.]
 01-Jan-1904 00:00:00 LT  .. 06-Feb-2040 06:28:15 LT  [MacOS Classic]
 (LT = local time)

Note that this package projects the Gregorian calendar back until the
year S<1 A.D.> -- even though the Gregorian calendar was only adopted
in 1582, mostly by the Catholic European countries, in obedience to the
corresponding decree of Pope S<Gregory XIII> in that year.

Some (mainly protestant) countries continued to use the Julian calendar
(used until then) until as late as the beginning of the 20th century.

Finally, note that this package is not intended to do everything you could
ever imagine automagically for you; it is rather intended to serve as a
toolbox (in the best of UNIX spirit and traditions) which should, however,
always get you where you want to go.

See the section "RECIPES" at the bottom of this document for solutions
to common problems!

If nevertheless you can't figure out how to solve a particular problem,
please let me know! (See e-mail address at the end of this document.)

=head1 SYNOPSIS

  use Date::Calc qw(
      Days_in_Year
      Days_in_Month
      Weeks_in_Year
      leap_year
      check_date
      check_time
      check_business_date
      Day_of_Year
      Date_to_Days
      Day_of_Week
      Week_Number
      Week_of_Year
      Monday_of_Week
      Nth_Weekday_of_Month_Year
      Standard_to_Business
      Business_to_Standard
      Delta_Days
      Delta_DHMS
      Delta_YMD
      Delta_YMDHMS
      Normalize_DHMS
      Add_Delta_Days
      Add_Delta_DHMS
      Add_Delta_YM
      Add_Delta_YMD
      Add_Delta_YMDHMS
      System_Clock
      Today
      Now
      Today_and_Now
      This_Year
      Gmtime
      Localtime
      Mktime
      Timezone
      Date_to_Time
      Time_to_Date
      Easter_Sunday
      Decode_Month
      Decode_Day_of_Week
      Decode_Language
      Decode_Date_EU
      Decode_Date_US
      Fixed_Window
      Moving_Window
      Compress
      Uncompress
      check_compressed
      Compressed_to_Text
      Date_to_Text
      Date_to_Text_Long
      English_Ordinal
      Calendar
      Month_to_Text
      Day_of_Week_to_Text
      Day_of_Week_Abbreviation
      Language_to_Text
      Language
      Languages
      Decode_Date_EU2
      Decode_Date_US2
      Parse_Date
      ISO_LC
      ISO_UC
  );

  use Date::Calc qw(:all);

  Days_in_Year
      $days = Days_in_Year($year,$month);

  Days_in_Month
      $days = Days_in_Month($year,$month);

  Weeks_in_Year
      $weeks = Weeks_in_Year($year);

  leap_year
      if (leap_year($year))

  check_date
      if (check_date($year,$month,$day))

  check_time
      if (check_time($hour,$min,$sec))

  check_business_date
      if (check_business_date($year,$week,$dow))

  Day_of_Year
      $doy = Day_of_Year($year,$month,$day);

  Date_to_Days
      $days = Date_to_Days($year,$month,$day);

  Day_of_Week
      $dow = Day_of_Week($year,$month,$day);

  Week_Number
      $week = Week_Number($year,$month,$day);          # DEPRECATED

  Week_of_Year
      ($week,$year) = Week_of_Year($year,$month,$day); # RECOMMENDED
      $week = Week_of_Year($year,$month,$day);         # DANGEROUS

  Monday_of_Week
      ($year,$month,$day) = Monday_of_Week($week,$year);

  Nth_Weekday_of_Month_Year
      if (($year,$month,$day) =
      Nth_Weekday_of_Month_Year($year,$month,$dow,$n))

  Standard_to_Business
      ($year,$week,$dow) =
      Standard_to_Business($year,$month,$day);

  Business_to_Standard
      ($year,$month,$day) =
      Business_to_Standard($year,$week,$dow);

  Delta_Days
      $Dd = Delta_Days($year1,$month1,$day1,
                       $year2,$month2,$day2);

  Delta_DHMS
      ($Dd,$Dh,$Dm,$Ds) =
      Delta_DHMS($year1,$month1,$day1, $hour1,$min1,$sec1,
                 $year2,$month2,$day2, $hour2,$min2,$sec2);

  Delta_YMD
      ($Dy,$Dm,$Dd) =
      Delta_YMD($year1,$month1,$day1,
                $year2,$month2,$day2);

  Delta_YMDHMS
      ($D_y,$D_m,$D_d, $Dh,$Dm,$Ds) =
      Delta_YMDHMS($year1,$month1,$day1, $hour1,$min1,$sec1,
                   $year2,$month2,$day2, $hour2,$min2,$sec2);

  Normalize_DHMS
      ($Dd,$Dh,$Dm,$Ds) =
      Normalize_DHMS($Dd,$Dh,$Dm,$Ds);

  Add_Delta_Days
      ($year,$month,$day) =
      Add_Delta_Days($year,$month,$day,
                     $Dd);

  Add_Delta_DHMS
      ($year,$month,$day, $hour,$min,$sec) =
      Add_Delta_DHMS($year,$month,$day, $hour,$min,$sec,
                     $Dd,$Dh,$Dm,$Ds);

  Add_Delta_YM
      ($year,$month,$day) =
      Add_Delta_YM($year,$month,$day,
                   $Dy,$Dm);

  Add_Delta_YMD
      ($year,$month,$day) =
      Add_Delta_YMD($year,$month,$day,
                    $Dy,$Dm,$Dd);

  Add_Delta_YMDHMS
      ($year,$month,$day, $hour,$min,$sec) =
      Add_Delta_YMDHMS($year,$month,$day, $hour,$min,$sec,
                       $D_y,$D_m,$D_d, $Dh,$Dm,$Ds);

  System_Clock
      ($year,$month,$day, $hour,$min,$sec, $doy,$dow,$dst) =
      System_Clock([$gmt]);

  Today
      ($year,$month,$day) = Today([$gmt]);

  Now
      ($hour,$min,$sec) = Now([$gmt]);

  Today_and_Now
      ($year,$month,$day, $hour,$min,$sec) = Today_and_Now([$gmt]);

  This_Year
      $year = This_Year([$gmt]);

  Gmtime
      ($year,$month,$day, $hour,$min,$sec, $doy,$dow,$dst) =
      Gmtime([time]);

  Localtime
      ($year,$month,$day, $hour,$min,$sec, $doy,$dow,$dst) =
      Localtime([time]);

  Mktime
      $time = Mktime($year,$month,$day, $hour,$min,$sec);

  Timezone
      ($D_y,$D_m,$D_d, $Dh,$Dm,$Ds, $dst) = Timezone([time]);

  Date_to_Time
      $time = Date_to_Time($year,$month,$day, $hour,$min,$sec);

  Time_to_Date
      ($year,$month,$day, $hour,$min,$sec) = Time_to_Date([time]);

  Easter_Sunday
      ($year,$month,$day) = Easter_Sunday($year);

  Decode_Month
      if ($month = Decode_Month($string))

  Decode_Day_of_Week
      if ($dow = Decode_Day_of_Week($string))

  Decode_Language
      if ($lang = Decode_Language($string))

  Decode_Date_EU
      if (($year,$month,$day) = Decode_Date_EU($string))

  Decode_Date_US
      if (($year,$month,$day) = Decode_Date_US($string))

  Fixed_Window
      $year = Fixed_Window($yy);

  Moving_Window
      $year = Moving_Window($yy);

  Compress
      $date = Compress($year,$month,$day);

  Uncompress
      if (($century,$year,$month,$day) = Uncompress($date))

  check_compressed
      if (check_compressed($date))

  Compressed_to_Text
      $string = Compressed_to_Text($date);

  Date_to_Text
      $string = Date_to_Text($year,$month,$day);

  Date_to_Text_Long
      $string = Date_to_Text_Long($year,$month,$day);

  English_Ordinal
      $string = English_Ordinal($number);

  Calendar
      $string = Calendar($year,$month[,$orthodox]);

  Month_to_Text
      $string = Month_to_Text($month);

  Day_of_Week_to_Text
      $string = Day_of_Week_to_Text($dow);

  Day_of_Week_Abbreviation
      $string = Day_of_Week_Abbreviation($dow);

  Language_to_Text
      $string = Language_to_Text($lang);

  Language
      $lang = Language();
      Language($lang);
      $oldlang = Language($newlang);

  Languages
      $max_lang = Languages();

  Decode_Date_EU2
      if (($year,$month,$day) = Decode_Date_EU2($string))

  Decode_Date_US2
      if (($year,$month,$day) = Decode_Date_US2($string))

  Parse_Date
      if (($year,$month,$day) = Parse_Date($string))

  ISO_LC
      $lower = ISO_LC($string);

  ISO_UC
      $upper = ISO_UC($string);

  Version
      $string = Date::Calc::Version();

=head1 IMPORTANT NOTES

(See the section "RECIPES" at the bottom of this document for
solutions to common problems!)

=over 2

=item *

"Year 2000" ("Y2K") compliance

The upper limit for any year number in this module is only given
by the size of the largest positive integer that can be represented
in a variable of the C type "int" on your system, which is at least
32767, according to the ANSI C standard (exceptions see below).

In order to simplify calculations, this module projects the gregorian
calendar back until the year S<1 A.D.> -- i.e., back B<BEYOND> the
year 1582 when this calendar was first decreed by the Catholic Pope
S<Gregory XIII>!

Therefore, B<BE SURE TO ALWAYS SPECIFY "1998" WHEN YOU MEAN "1998">,
for instance, and B<DO NOT WRITE "98" INSTEAD>, because this will
in fact perform a calculation based on the year "98" A.D. and
B<NOT> "1998"!

An exception from this rule are the functions which contain the
word "compress" in their names (which can only handle years between
1970 and 2069 and also accept the abbreviations "00" to "99"), and
the functions whose names begin with "Decode_Date_" (which translate
year numbers below 100 using a technique known as "moving window").

If you want to convert a two-digit year number into a full-fledged,
four-digit (at least for some years to come C<;-)>) year number,
use the two functions "Fixed_Window()" and "Moving_Window()"
(see their description further below).

Note also that the following import/export functions (which are
interfaces to the POSIX functions "time()", "gmtime()", "localtime()"
and "mktime()" or (the last two) substitutes for the BSD function
"timegm()" and the POSIX function "gmtime()") have a very limited
range of representable dates (in contrast to all other functions
in this package, which cover virtually any date including and
after S<January 1st 1 A.D.>):

              System_Clock()
              Today()
              Now()
              Today_and_Now()
              This_Year()
              Gmtime()
              Localtime()
              Mktime()
              Timezone()
              Date_to_Time()
              Time_to_Date()

These functions can only deal with dates in the range from
S<01-Jan-1970 00:00:00 GMT> to S<19-Jan-2038 03:14:07 GMT>
(the latter limit is only authoritative on S<32 bit> systems,
however, and can (in principle, through a few code changes)
be extended somewhat C<:-)> on S<64 bit> systems).

On MacOS Classic, the valid range of dates is between
(both included) S<01-Jan-1904 00:00:00> (local time)
to S<06-Feb-2040 06:28:15> (local time).

Note further that the function "Easter_Sunday()" can only
be used for years in the range 1583 to 2299.

=item *

First index

B<ALL> ranges in this module start with "C<1>", B<NOT> "C<0>"!

I.e., the day of month, day of week, day of year, month of year,
week of year, first valid year number and language B<ALL> start
counting at one, B<NOT> zero!

The only exception is the function "C<Week_Number()>", which may
in fact return "C<0>" when the given date actually lies in the
last week of the B<PREVIOUS> year, and of course the numbers for
hours (C<0..23>), minutes (C<0..59>) and seconds (C<0..59>).

=item *

Function naming conventions

Function names completely in lower case indicate a boolean return value.

=item *

Boolean values

Boolean values in this module are always a numeric zero ("C<0>") for
"false" and a numeric one ("C<1>") for "true".

=item *

Exception handling

The functions in this module will usually die with a corresponding error
message if their input parameters, intermediate results or output values
are out of range.

The following functions handle errors differently:

  -  check_date()
  -  check_time()
  -  check_business_date()
  -  check_compressed()

(which return a "false" return value when the given input does not represent
a valid date or time),

  -  Nth_Weekday_of_Month_Year()

(which returns an empty list if the requested 5th day of week does not exist),

  -  Decode_Month()
  -  Decode_Day_of_Week()
  -  Decode_Language()
  -  Fixed_Window()
  -  Moving_Window()
  -  Compress()

(which return "C<0>" upon failure or invalid input), and

  -  Decode_Date_EU()
  -  Decode_Date_US()
  -  Decode_Date_EU2()
  -  Decode_Date_US2()
  -  Parse_Date()
  -  Uncompress()

(which return an empty list upon failure or invalid input).

Note that you can always catch an exception thrown by any of the functions
in this module and handle it yourself by enclosing the function call in an
"C<eval>" with curly brackets and checking the special variable "C<$@>"
(see L<perlfunc(1)/eval> for details).

=back

=head1 DESCRIPTION

=over 2

=item *

C<use Date::Calc qw( Days_in_Year Days_in_Month ... );>

=item *

C<use Date::Calc qw(:all);>

You can either specify the functions you want to import explicitly by
enumerating them between the parentheses of the "C<qw()>" operator, or
you can use the "C<:all>" tag instead to import B<ALL> available functions.

=item *

C<$days = Days_in_Year($year,$month);>

This function returns the sum of the number of days in the months starting
with January up to and including "C<$month>" in the given year "C<$year>".

I.e., "C<Days_in_Year(1998,1)>" returns "C<31>", "C<Days_in_Year(1998,2)>"
returns "C<59>", "C<Days_in_Year(1998,3)>" returns "C<90>", and so on.

Note that "C<Days_in_Year($year,12)>" returns the number of days in the
given year "C<$year>", i.e., either "C<365>" or "C<366>".

=item *

C<$days = Days_in_Month($year,$month);>

This function returns the number of days in the given month "C<$month>" of
the given year "C<$year>".

The year must always be supplied, even though it is only needed when the
month is February, in order to determine whether it is a leap year or not.

I.e., "C<Days_in_Month(1998,1)>" returns "C<31>", "C<Days_in_Month(1998,2)>"
returns "C<28>", "C<Days_in_Month(2000,2)>" returns "C<29>",
"C<Days_in_Month(1998,3)>" returns "C<31>", and so on.

=item *

C<$weeks = Weeks_in_Year($year);>

This function returns the number of weeks in the given year "C<$year>",
i.e., either "C<52>" or "C<53>".

=item *

C<if (leap_year($year))>

This function returns "true" ("C<1>") if the given year "C<$year>" is
a leap year and "false" ("C<0>") otherwise.

=item *

C<if (check_date($year,$month,$day))>

This function returns "true" ("C<1>") if the given three numerical
values "C<$year>", "C<$month>" and "C<$day>" constitute a valid date,
and "false" ("C<0>") otherwise.

=item *

C<if (check_time($hour,$min,$sec))>

This function returns "true" ("C<1>") if the given three numerical
values "C<$hour>", "C<$min>" and "C<$sec>" constitute a valid time
(C<0 E<lt>= $hour E<lt> 24>, C<0 E<lt>= $min E<lt> 60> and
C<0 E<lt>= $sec E<lt> 60>), and "false" ("C<0>") otherwise.

=item *

C<if (check_business_date($year,$week,$dow))>

This function returns "true" ("C<1>") if the given three numerical
values "C<$year>", "C<$week>" and "C<$dow>" constitute a valid date
in business format, and "false" ("C<0>") otherwise.

B<Beware> that this function does B<NOT> compute whether a given date
is a business day (i.e., Monday to Friday)!

To do so, use "C<(Day_of_Week($year,$month,$day) E<lt> 6)>" instead.

=item *

C<$doy = Day_of_Year($year,$month,$day);>

This function returns the (relative) number of the day of the given date
in the given year.

E.g., "C<Day_of_Year($year,1,1)>" returns "C<1>",
"C<Day_of_Year($year,2,1)>" returns "C<32>", and
"C<Day_of_Year($year,12,31)>" returns either "C<365>" or "C<366>".

=item *

C<$days = Date_to_Days($year,$month,$day);>

This function returns the (absolute) number of the day of the given date,
where counting starts at the 1st of January of the year S<1 A.D.>

I.e., "C<Date_to_Days(1,1,1)>" returns "C<1>", "C<Date_to_Days(1,12,31)>"
returns "C<365>", "C<Date_to_Days(2,1,1)>" returns "C<366>",
"C<Date_to_Days(1998,5,1)>" returns "C<729510>", and so on.

This is sometimes also referred to (not quite correctly) as the Julian
date (or day). This may cause confusion, because also the number of the
day in a year (from 1 to 365 or 366) is frequently called the "Julian date".

In fact the calendar that was used B<BEFORE> the Gregorian calendar
was the Julian calendar - named after famous Julius Caesar, who had
instituted it in Roman times. The Julian calendar was less precise
because it had too many leap years compared to the true mean length
of a year, and because rulers often changed it arbitrarily, in order
to lengthen their own reign, for instance.

In order to convert the number returned by this function back into
a date, use the function "C<Add_Delta_Days()>" (described further
below), as follows:

  $days = Date_to_Days($year,$month,$day);
  ($year,$month,$day) = Add_Delta_Days(1,1,1, $days - 1);

=item *

C<$dow = Day_of_Week($year,$month,$day);>

This function returns the number of the day of week of the given date.

The function returns "C<1>" for Monday, "C<2>" for Tuesday and so on
until "C<7>" for Sunday.

Note that in the Hebrew calendar (on which the Christian calendar is based),
the week starts with Sunday and ends with the Sabbath or Saturday (where
according to the Genesis (as described in the Bible) the Lord rested from
creating the world).

In medieval times, Catholic Popes have decreed the Sunday to be the official
day of rest, in order to dissociate the Christian from the Hebrew belief.

Nowadays, the Sunday B<AND> the Saturday are commonly considered (and
used as) days of rest, usually referred to as the "week-end".

Consistent with this practice, current norms and standards (such as
S<ISO/R 2015-1971>, S<DIN 1355> and S<ISO 8601>) define the Monday
as the first day of the week.

=item *

C<$week = Week_Number($year,$month,$day);>

This function returns the number of the week the given date lies in.

If the given date lies in the B<LAST> week of the B<PREVIOUS> year,
"C<0>" is returned.

If the given date lies in the B<FIRST> week of the B<NEXT> year,
"C<Weeks_in_Year($year) + 1>" is returned.

=item *

C<($week,$year) = Week_of_Year($year,$month,$day);>

This function returns the number of the week the given date lies in,
as well as the year that week belongs to.

I.e., if the given date lies in the B<LAST> week of the B<PREVIOUS> year,
"C<(Weeks_in_Year($year-1), $year-1)>" is returned.

If the given date lies in the B<FIRST> week of the B<NEXT> year,
"C<(1, $year+1)>" is returned.

Otherwise, "C<(Week_Number($year,$month,$day), $year)>" is returned.

=item *

C<$week = Week_of_Year($year,$month,$day);>

In scalar context, this function returns just the week number. This
allows you to write "C<$week = Week_of_Year($year,$month,$day);>"
instead of "C<($week) = Week_of_Year($year,$month,$day);>" (note
the parentheses around "C<$week>").

If the given date lies in the B<LAST> week of the B<PREVIOUS> year,
"C<Weeks_in_Year($year-1)>" is returned.

If the given date lies in the B<FIRST> week of the B<NEXT> year,
"C<1>" is returned.

Otherwise the return value is identical with that of
"C<Week_Number($year,$month,$day)>".

B<BEWARE> that using this function in scalar context is a B<DANGEROUS>
feature, because without knowing which year the week belongs to, you
might inadvertently assume the wrong one!

If for instance you are iterating through an interval of dates, you might
assume that the week always belongs to the same year as the given date,
which unfortunately is B<WRONG> in some cases!

In many years, the 31st of December for instance belongs to week number
one of the B<FOLLOWING> year. Assuming that the year is the same as your
date (31st of December, in this example), sends you back to the first week
of the B<CURRENT> year - the Monday of which, by the way, in case of bad
luck, might actually lie in the year B<BEFORE> the current year!

This actually happens in 2002, for example.

So you always need to provide the correct corresponding year number
by other means, keeping track of it yourself.

In case you do not understand this, never mind, but then simply
B<DO NOT USE> this function in scalar context!

=item *

C<($year,$month,$day) = Monday_of_Week($week,$year);>

This function returns the date of the first day of the given week, i.e.,
the Monday.

"C<$year>" must be greater than or equal to "C<1>", and "C<$week>" must
lie in the range "C<1>" to "C<Weeks_in_Year($year)>".

Note that you can write
"C<($year,$month,$day) = Monday_of_Week(Week_of_Year($year,$month,$day));>"
in order to calculate the date of the Monday of the same week as the
given date.

If you want to calculate any other day of week in the same week as a
given date, use

  @date = Add_Delta_Days(Monday_of_Week(Week_of_Year(@date)),$offset);

where C<$offset = 1> for Tuesday, C<2> for Wednesday etc.

=item *

C<if (($year,$month,$day) = Nth_Weekday_of_Month_Year($year,$month,$dow,$n))>

This function calculates the date of the "C<$n>"th day of week "C<$dow>"
in the given month "C<$month>" and year "C<$year>"; such as, for example,
the 3rd Thursday of a given month and year.

This can be used to send a notification mail to the members of a group
which meets regularly on every 3rd Thursday of a month, for instance.

(See the section "RECIPES" near the end of this document for a code
snippet to actually do so.)

"C<$year>" must be greater than or equal to "C<1>", "C<$month>" must lie
in the range "C<1>" to "C<12>", "C<$dow>" must lie in the range "C<1>"
to "C<7>" and "C<$n>" must lie in the range "C<1>" to "C<5>", or a fatal
error (with appropriate error message) occurs.

The function returns an empty list when the 5th of a given day of week
does not exist in the given month and year.

=item *

C<($year,$week,$dow) = Standard_to_Business($year,$month,$day);>

This function converts a given date from standard notation (year,
month, day (of month)) to business notation (year, week, day of week).

=item *

C<($year,$month,$day) = Business_to_Standard($year,$week,$dow);>

This function converts a given date from business notation (year,
week, day of week) to standard notation (year, month, day (of month)).

=item *

C<$Dd = Delta_Days($year1,$month1,$day1, $year2,$month2,$day2);>

This function returns the difference in days between the two given
dates.

The result is positive if the two dates are in chronological order,
i.e., if date #1 comes chronologically B<BEFORE> date #2, and negative
if the order of the two dates is reversed.

The result is zero if the two dates are identical.

=item *

C<($Dd,$Dh,$Dm,$Ds) = Delta_DHMS($year1,$month1,$day1, $hour1,$min1,$sec1, $year2,$month2,$day2, $hour2,$min2,$sec2);>

This function returns the difference in days, hours, minutes and seconds
between the two given dates with times.

All four return values will be positive if the two dates are in chronological
order, i.e., if date #1 comes chronologically B<BEFORE> date #2, and negative
(in all four return values!) if the order of the two dates is reversed.

This is so that the two functions "C<Delta_DHMS()>" and "C<Add_Delta_DHMS()>"
(description see further below) are complementary, i.e., mutually inverse:

  Add_Delta_DHMS(@date1,@time1, Delta_DHMS(@date1,@time1, @date2,@time2))

yields "C<(@date2,@time2)>" again, whereas

  Add_Delta_DHMS(@date2,@time2,
      map(-$_, Delta_DHMS(@date1,@time1, @date2,@time2)))

yields "C<(@date1,@time1)>", and

  Delta_DHMS(@date1,@time1, Add_Delta_DHMS(@date1,@time1, @delta))

yields "C<@delta>" again.

The result is zero (in all four return values) if the two dates and times
are identical.

=item *

C<($Dy,$Dm,$Dd) = Delta_YMD($year1,$month1,$day1, $year2,$month2,$day2);>

This function returns the vector

    ( $year2 - $year1, $month2 - $month1, $day2 - $day1 )

An error occurs if any of the two dates is invalid.

=item *

C<($D_y,$D_m,$D_d, $Dh,$Dm,$Ds) = Delta_YMDHMS($year1,$month1,$day1, $hour1,$min1,$sec1, $year2,$month2,$day2, $hour2,$min2,$sec2);>

This function is based on the function "Delta_YMD()" above but additionally
calculates the time difference. When a carry over from the time difference
occurs, the value of "C<$D_d>" is adjusted accordingly, thus giving the
correct total date/time difference.

Arguments are expected to be in chronological order to yield a (usually)
positive result.

In any case, adding the result of this function to the first date/time value
(C<$year1,$month1,$day1,> C<$hour1,$min1,$sec1>) always gives the second
date/time value (C<$year2,$month2,$day2,> C<$hour2,$min2,$sec2>) again,
and adding the negative result (all elements of the result vector negated)
to the second date/time value gives the first date/time value.

See the function "Add_Delta_YMDHMS()" further below for adding a date/time
value and a date/time difference.

An error occurs if any of the two date/time values is invalid.

=item *

C<($Dd,$Dh,$Dm,$Ds) = Normalize_DHMS($Dd,$Dh,$Dm,$Ds);>

This function takes four arbitrary values for days, hours, minutes
and seconds (which may have different signs) and renormalizes them
so that the values for hours, minutes and seconds will lie in the
ranges C<[-23..23]>, C<[-59..59]> and C<[-59..59]>, respectively,
and so that all four values have the same sign (or are zero).

The given values are left untouched, i.e., unchanged.

=item *

C<($year,$month,$day) = Add_Delta_Days($year,$month,$day, $Dd);>

This function has two principal uses:

First, it can be used to calculate a new date, given an initial date and
an offset (which may be positive or negative) in days, in order to answer
questions like "today plus 90 days -- which date gives that?".

(In order to add a weeks offset, simply multiply the weeks offset with
"C<7>" and use that as your days offset.)

Second, it can be used to convert the canonical representation of a date,
i.e., the number of that day (where counting starts at the 1st of January
in S<1 A.D.>), back into a date given as year, month and day.

Because counting starts at "C<1>", you will actually have to subtract "C<1>"
from the canonical date in order to get back the original date:

  $canonical = Date_to_Days($year,$month,$day);

  ($year,$month,$day) = Add_Delta_Days(1,1,1, $canonical - 1);

Moreover, this function is the inverse of the function "C<Delta_Days()>":

  Add_Delta_Days(@date1, Delta_Days(@date1, @date2))

yields "C<@date2>" again, whereas

  Add_Delta_Days(@date2, -Delta_Days(@date1, @date2))

yields "C<@date1>", and

  Delta_Days(@date1, Add_Delta_Days(@date1, $delta))

yields "C<$delta>" again.

=item *

C<($year,$month,$day, $hour,$min,$sec) = Add_Delta_DHMS($year,$month,$day, $hour,$min,$sec, $Dd,$Dh,$Dm,$Ds);>

This function serves to add a days, hours, minutes and seconds offset to a
given date and time, in order to answer questions like "today and now plus
7 days but minus 5 hours and then plus 30 minutes, what date and time gives
that?":

  ($y,$m,$d,$H,$M,$S) = Add_Delta_DHMS(Today_and_Now(), +7,-5,+30,0);

=item *

C<($year,$month,$day) = Add_Delta_YM($year,$month,$day, $Dy,$Dm);>

This function can be used to add a year and/or month offset to a given
date.

In contrast to the function described immediately below
("C<Add_Delta_YMD()>"), this function does no "wrapping" into
the next month if the day happens to lie outside the valid range
for the resulting year and month (after adding the year and month
offsets). Instead, it simply truncates the day to the last possible
day of the resulting month.

Examples:

Adding an offset of 0 years, 1 month to the date (1999,1,31) would result
in the (invalid) date (1999,2,31). The function replaces this result by
the (valid) date (1999,2,28).

Adding an offset of 1 year, 1 month to the same date (1999,1,31) as above
would result in the (still invalid) date (2000,2,31). The function replaces
this result by the valid date (2000,2,29) (because 2000 is a leap year).

Note that the year and month offsets can be negative, and that they can
have different signs.

If you want to additionally add a days offset, use the function
"C<Add_Delta_Days()>" before or after calling "C<Add_Delta_YM()>":

  @date2 = Add_Delta_Days( Add_Delta_YM(@date1, $Dy,$Dm), $Dd );
  @date2 = Add_Delta_YM( Add_Delta_Days(@date1, $Dd), $Dy,$Dm );

Note that your result may depend on the order in which you call
these two functions!

Consider the date (1999,2,28) and the offsets 0 years, 1 month
and 1 day:

(1999,2,28) plus one month is (1999,3,28), plus one day is
(1999,3,29). (1999,2,28) plus one day is (1999,3,1), plus
one month is (1999,4,1).

(Which is also the reason why the "C<Add_Delta_YM()>" function
does not allow to add a days offset, because this would actually
require TWO functions: One for adding the days offset BEFORE and
one for adding it AFTER applying the year/month offsets.)

An error occurs if the initial date is not valid.

Note that "C<Add_Delta_YM( Add_Delta_YM(@date, $Dy,$Dm), -$Dy,-$Dm );>"
will not, in general, return the original date "C<@date>" (consider
the examples given above!).

=item *

C<($year,$month,$day) = Add_Delta_YMD($year,$month,$day, $Dy,$Dm,$Dd);>

This function serves to add a years, months and days offset to a given date.

(In order to add a weeks offset, simply multiply the weeks offset with "C<7>"
and add this number to your days offset.)

Note that the three offsets for years, months and days are applied
independently from each other. This also allows them to have
different signs.

The years and months offsets are applied first, and the days offset
is applied last.

If the resulting date happens to fall on a day after the end of the
resulting month, like the 32nd of April or the 30th of February, then
the date is simply counted forward into the next month (possibly also
into the next year) by the number of excessive days (e.g., the 32nd of
April will become the 2nd of May).

B<BEWARE> that this behaviour differs from that of previous versions
of this module! In previous versions, the day was simply truncated to
the maximum number of days in the resulting month.

If you want the previous behaviour, use the new function "C<Add_Delta_YM()>"
(described immediately above) plus the function "C<Add_Delta_Days()>"
instead.

B<BEWARE> also that because a year and a month offset is not equivalent
to a fixed number of days, the transformation performed by this function
is B<NOT ALWAYS REVERSIBLE>!

This is in contrast to the functions "C<Add_Delta_Days()>" and
"C<Add_Delta_DHMS()>", which are fully and truly reversible (with
the help of the functions "C<Delta_Days()>" and "C<Delta_DHMS()>",
for instance).

Note that for this same reason,

  @date = Add_Delta_YMD(
          Add_Delta_YMD(@date, $Dy,$Dm,$Dd), -$Dy,-$Dm,-$Dd);

will in general B<NOT> return the initial date "C<@date>".

Note that this is B<NOT> a program bug but B<NECESSARILY> so
because of the variable lengths of years and months!

=item *

C<($year,$month,$day, $hour,$min,$sec) = Add_Delta_YMDHMS($year,$month,$day, $hour,$min,$sec, $D_y,$D_m,$D_d, $Dh,$Dm,$Ds);>

Same as the function above, except that a time offset may be given in
addition to the year, month and day offset.

=item *

C<($year,$month,$day, $hour,$min,$sec, $doy,$dow,$dst) = System_Clock([$gmt]);>

If your operating system supports the corresponding system calls
("C<time()>" and "C<localtime()>" or "C<gmtime()>"), this function
will return the information provided by your system clock, i.e.,
the current date and time, the number of the day of year, the number
of the day of week and a flag signaling whether daylight savings time
is currently in effect or not.

The ranges of values returned (and their meanings) are as follows:

        $year   :   1970..2038 (or more)  [Unix etc.]
        $year   :   1904..2040            [MacOS Classic]

        $month  :   1..12
        $day    :   1..31
        $hour   :   0..23
        $min    :   0..59
        $sec    :   0..59    (0..61 on some systems)
        $doy    :   1..366
        $dow    :   1..7
        $dst    :  -1..1

"C<$doy>" is the day of year, sometimes also referred to as the
"julian date", which starts at "C<1>" and goes up to the number
of days in that year.

The day of week ("C<$dow>") will be "C<1>" for Monday, "C<2>" for
Tuesday and so on until "C<7>" for Sunday.

The daylight savings time flag ("C<$dst>") will be "C<-1>" if this
information is not available on your system, "C<0>" for no daylight
savings time (i.e., winter time) and "C<1>" when daylight savings
time is in effect.

If your operating system does not provide the necessary system calls,
calling this function will result in a fatal "not available on this
system" error message.

If you want to handle this exception yourself, use "C<eval>" as follows:

  eval { ($year,$month,$day, $hour,$min,$sec, $doy,$dow,$dst) =
    System_Clock(); };

  if ($@)
  {
      # Handle missing system clock
      # (For instance, ask user to enter this information manually)
  }

Note that curlies ("{" and "}") are used here to delimit the statement to
be "eval"ed (which is the way to catch exceptions in Perl), and not quotes
(which is a way to evaluate Perl expressions at runtime).

If the optional (boolean) input parameter "C<$gmt>" is given, a "true"
value ("C<1>") will cause "C<gmtime()>" to be used instead of "C<localtime()>",
internally, thus returning Greenwich Mean Time (GMT, or UTC) instead of
local time.

=item *

C<($year,$month,$day) = Today([$gmt]);>

This function returns a subset of the values returned by the function
"C<System_Clock()>" (see above for details), namely the current year,
month and day.

A fatal "not available on this system" error message will appear if the
corresponding system calls are not supported by your current operating
system.

If the optional (boolean) input parameter "C<$gmt>" is given, a "true"
value ("C<1>") will cause "C<gmtime()>" to be used instead of "C<localtime()>",
internally, thus returning Greenwich Mean Time (GMT, or UTC) instead of
local time.

=item *

C<($hour,$min,$sec) = Now([$gmt]);>

This function returns a subset of the values returned by the function
"C<System_Clock()>" (see above for details), namely the current time
(hours, minutes and full seconds).

A fatal "not available on this system" error message will appear if the
corresponding system calls are not supported by your current operating
system.

If the optional (boolean) input parameter "C<$gmt>" is given, a "true"
value ("C<1>") will cause "C<gmtime()>" to be used instead of "C<localtime()>",
internally, thus returning Greenwich Mean Time (GMT, or UTC) instead of
local time.

=item *

C<($year,$month,$day, $hour,$min,$sec) = Today_and_Now([$gmt]);>

This function returns a subset of the values returned by the function
"C<System_Clock()>" (see above for details), namely the current date
(year, month, day) and time (hours, minutes and full seconds).

A fatal "not available on this system" error message will appear if the
corresponding system calls are not supported by your current operating
system.

If the optional (boolean) input parameter "C<$gmt>" is given, a "true"
value ("C<1>") will cause "C<gmtime()>" to be used instead of "C<localtime()>",
internally, thus returning Greenwich Mean Time (GMT, or UTC) instead of
local time.

=item *

C<$year = This_Year([$gmt]);>

This function returns the current year, according to local time.

A fatal "not available on this system" error message will appear if the
corresponding system calls are not supported by your current operating
system.

If the optional (boolean) input parameter "C<$gmt>" is given, a "true"
value ("C<1>") will cause "C<gmtime()>" to be used instead of "C<localtime()>",
internally, thus returning Greenwich Mean Time (GMT, or UTC) instead of
local time. However, this will only make a difference within a few hours
around New Year (unless you are on a Pacific island, where this can
be almost 24 hours).

=item *

C<($year,$month,$day, $hour,$min,$sec, $doy,$dow,$dst) = Gmtime([time]);>

This is Date::Calc's equivalent of Perl's built-in "gmtime()" function.
See also L<perlfunc/gmtime>.

The ranges of values returned (and their meanings) are as follows:

        $year   :   1970..2038 (or more)  [Unix etc.]
        $year   :   1904..2040            [MacOS Classic]

        $month  :   1..12
        $day    :   1..31
        $hour   :   0..23
        $min    :   0..59
        $sec    :   0..59
        $doy    :   1..366
        $dow    :   1..7
        $dst    :  -1..1

"C<$doy>" is the day of year, sometimes also referred to as the
"julian date", which starts at "C<1>" and goes up to the number
of days in that year.

The day of week ("C<$dow>") will be "C<1>" for Monday, "C<2>" for
Tuesday and so on until "C<7>" for Sunday.

The daylight savings time flag ("C<$dst>") will be "C<-1>" if this
information is not available on your system, "C<0>" for no daylight
savings time (i.e., winter time) and "C<1>" when daylight savings
time is in effect.

A fatal "time out of range" error will occur if the given time value
is out of range C<[0..(~0E<gt>E<gt>1)]>.

If the time value is omitted, the "time()" function is called instead,
internally.

=item *

C<($year,$month,$day, $hour,$min,$sec, $doy,$dow,$dst) = Localtime([time]);>

This is Date::Calc's equivalent of Perl's built-in "localtime()" function.
See also L<perlfunc/localtime>.

The ranges of values returned (and their meanings) are as follows:

        $year   :   1970..2038 (or more)  [Unix etc.]
        $year   :   1904..2040            [MacOS Classic]

        $month  :   1..12
        $day    :   1..31
        $hour   :   0..23
        $min    :   0..59
        $sec    :   0..59
        $doy    :   1..366
        $dow    :   1..7
        $dst    :  -1..1

"C<$doy>" is the day of year, sometimes also referred to as the
"julian date", which starts at "C<1>" and goes up to the number
of days in that year.

The day of week ("C<$dow>") will be "C<1>" for Monday, "C<2>" for
Tuesday and so on until "C<7>" for Sunday.

The daylight savings time flag ("C<$dst>") will be "C<-1>" if this
information is not available on your system, "C<0>" for no daylight
savings time (i.e., winter time) and "C<1>" when daylight savings
time is in effect.

A fatal "time out of range" error will occur if the given time value is
out of range C<[0..(~0E<gt>E<gt>1)]>.

If the time value is omitted, the "time()" function is called instead,
internally.

=item *

C<$time = Mktime($year,$month,$day, $hour,$min,$sec);>

This function converts a date into a time value, i.e., into the number
of seconds since whatever moment in time your system considers to be
the "epoch". On Unix and most other systems this is the number of seconds
since January 1st 1970 at midnight (GMT). On MacOS Classic this is the
number of seconds since January 1st 1904 at midnight (local time).

The function is similar to the "POSIX::mktime()" function (see L<POSIX/mktime>
for more details), but in contrast to the latter, it expects dates in the
usual ranges used throughout this module: The year 2001 stays year 2001,
and months are numbered from 1 to 12.

A fatal "date out of range" error will occur if the given date cannot
be expressed in terms of seconds since the epoch (this happens for
instance when the date lies before the epoch, or if it is later than
S<19-Jan-2038 03:14:07 GMT> on S<32 bit> Unix systems, or later than
S<06-Feb-2040 06:28:15> (local time) on a Macintosh with MacOS Classic).

Just like the "POSIX::mktime()" function, this function uses the
"mktime()" system call, internally.

This means that the given date and time is considered to be in local time,
and that the value returned by this function will depend on your machine's
local settings such as the time zone, whether daylight savings time is
(or was, at the time) in effect, and the system clock itself.

B<BEWARE> that "mktime()" does not always return the same time value
as fed into "localtime()", when you feed the output of "localtime()"
back into "mktime()", on some systems!

I.e., "C<Mktime((Localtime($time))[0..5])>" will not always return
the same value as given in "C<$time>"!

=item *

C<($D_y,$D_m,$D_d, $Dh,$Dm,$Ds, $dst) = Timezone([time]);>

This function returns the difference between "C<localtime(time)>" and
"C<gmtime(time)>", which is the timezone offset in effect for the current
location and the given "C<time>".

This offset is positive if you are located to the east of Greenwich,
and is usually negative (except during daylight savings time, in some
locations) if you are located to the west of Greenwich.

Note that this offset is influenced by all of the relevant system
settings and parameters on your machine; such as locales, environment
variables (e.g. "C<TZ>") and the system clock itself. See the
relevant documentation on your system for more details.

If the "C<time>" is omitted, the "C<time()>" function will
be called automatically, internally (similar to the built-in
functions "C<localtime()>" and "C<gmtime()>" in Perl).

A fatal "time out of range" error will occur if the given time value
is out of range C<[0..(~0E<gt>E<gt>1)]>.

The last item of the returned list is a flag which indicates whether
daylight savings time is currently in effect. This flag is negative
(-1) if this information is not available on your system. It is zero
(0) when daylight savings time is off, and positive (+1) when daylight
savings time is on.

Thus you can check very quickly whether daylight savings time is
currently in effect by evaluating this function in scalar context
(in scalar context, Perl returns the last item of a list):

  if (scalar Timezone > 0) { # yes, daylight savings time

However, a slightly more efficient way would be this:

  if (scalar System_Clock > 0) { # yes, daylight savings time

=item *

C<$time = Date_to_Time($year,$month,$day, $hour,$min,$sec);>

This function is a replacement for the BSD function "timegm()"
(which is not available on all Unix systems), which converts
a given date and time into a time value, i.e., into the number
of seconds since whatever moment in time your system considers to be
the "epoch". On Unix and most other systems this is the number of seconds
since January 1st 1970 at midnight (GMT). On MacOS Classic this is the
number of seconds since January 1st 1904 at midnight (local time).

Under Unix, the date and time are considered to be in UTC
("Universal Time Coordinated", and so is the resulting time value.

UTC is almost the same as GMT (or "Greenwich Mean Time"), except
that UTC has leap seconds (in order to account for small variations
in the rotation of the earth, for instance), whereas GMT does not.

Under MacOS Classic, however, both input and output are
considered to be in local time.

The ranges of year and month follow the same rules as throughout
the rest of this module (and not the contorted rules of its Unix
equivalent), i.e., the year "2001" stays "2001" and the month
ranges from 1 to 12.

A fatal "date out of range" error will occur if the given date cannot
be expressed in terms of seconds since the epoch (this happens for
instance when the date lies before the epoch, or if it is later than
S<19-Jan-2038 03:14:07 GMT> on S<32 bit> Unix systems, or later than
S<06-Feb-2040 06:28:15> (local time) on a Macintosh with MacOS Classic).

This function should be very fast, because it is implemented in
a very straightforward manner and doesn't use any internal system
calls.

Moreover, the functions "Date_to_Time()" and "Time_to_Date()"
are guaranteed to be complementary, i.e., that
"C<Date_to_Time(Time_to_Date($time))>" and
"C<Time_to_Date(Date_to_Time($year,$month,$day, $hour,$min,$sec))>"
will always return the initial values.

=item *

C<($year,$month,$day, $hour,$min,$sec) = Time_to_Date([time]);>

This function is an alternative to the POSIX "gmtime()" function
(and its built-in Perl equivalent), which converts a given time
value into the corresponding date and time. The given time value
must be the number of seconds since whatever moment in time your
system considers to be the "epoch". On Unix and most other systems
this is the number of seconds since January 1st 1970 at midnight
(GMT). On MacOS Classic this is the number of seconds since
January 1st 1904 at midnight (local time).

Under Unix, the given time value is considered to be in UTC
("Universal Time Coordinated", and so is the resulting date
and time.

UTC is almost the same as GMT (or "Greenwich Mean Time"), except
that UTC has leap seconds (in order to account for small variations
in the rotation of the earth, for instance), whereas GMT does not.

Under MacOS Classic, however, both input and output
are considered to be in local time.

If the input value "C<time>" is omitted, the "C<time()>" function
will be called automatically, internally (similar to the built-in
functions "C<localtime()>" and "C<gmtime()>" in Perl).

A fatal "time out of range" error will occur if the given time
value is negative.

This function should be very fast, because it is implemented in
a very straightforward manner and doesn't use any internal system
calls (except for "time()", if the input value is omitted).

Moreover, the functions "Date_to_Time()" and "Time_to_Date()"
are guaranteed to be complementary, i.e., that
"C<Date_to_Time(Time_to_Date($time))>" and
"C<Time_to_Date(Date_to_Time($year,$month,$day, $hour,$min,$sec))>"
will always return the initial values.

=item *

C<($year,$month,$day) = Easter_Sunday($year);>

This function calculates the date of Easter Sunday for all years in the
range from 1583 to 2299 (all other year numbers will result in a fatal
"year out of range" error message) using the method known as the "Gaussian
Rule".

Some related christian feast days which depend on the date of Easter Sunday:

  Carnival Monday / Rosenmontag / Veille du Mardi Gras   =  -48 days
  Mardi Gras / Karnevalsdienstag / Mardi Gras            =  -47 days
  Ash Wednesday / Aschermittwoch / Mercredi des Cendres  =  -46 days
  Palm Sunday / Palmsonntag / Dimanche des Rameaux       =   -7 days
  Easter Friday / Karfreitag / Vendredi Saint            =   -2 days
  Easter Saturday / Ostersamstag / Samedi de Paques      =   -1 day
  Easter Monday / Ostermontag / Lundi de Paques          =   +1 day
  Ascension of Christ / Christi Himmelfahrt / Ascension  =  +39 days
  Whitsunday / Pfingstsonntag / Dimanche de Pentecote    =  +49 days
  Whitmonday / Pfingstmontag / Lundi de Pentecote        =  +50 days
  Feast of Corpus Christi / Fronleichnam / Fete-Dieu     =  +60 days

Use the offsets shown above to calculate the date of the corresponding
feast day as follows:

  ($year,$month,$day) = Add_Delta_Days(Easter_Sunday($year), $offset));

=item *

C<if ($month = Decode_Month($string))>

This function takes a string as its argument, which should contain the
name of a month B<IN THE CURRENTLY SELECTED LANGUAGE> (see further below
for details about the multi-language support of this package), or any uniquely
identifying abbreviation of a month's name (i.e., the first few letters),
and returns the corresponding number (1..12) upon a successful match, or
"C<0>" otherwise (therefore, the return value can also be used as the
conditional expression in an "if" statement).

Note that the input string may not contain any other characters which do not
pertain to the month's name, especially no leading or trailing whitespace.

Note also that matching is performed in a case-insensitive manner (this may
depend on the "locale" setting on your current system, though!)

With "English" as the currently selected language (which is the default),
the following examples will all return the value "C<9>":

  $month = Decode_Month("s");
  $month = Decode_Month("Sep");
  $month = Decode_Month("septemb");
  $month = Decode_Month("September");

=item *

C<if ($dow = Decode_Day_of_Week($string))>

This function takes a string as its argument, which should contain the
name of a day of week B<IN THE CURRENTLY SELECTED LANGUAGE> (see further
below for details about the multi-language support of this package), or any
uniquely identifying abbreviation of the name of a day of week (i.e., the
first few letters), and returns the corresponding number (1..7) upon a
successful match, or "C<0>" otherwise (therefore, the return value can
also be used as the conditional expression in an "if" statement).

Note that the input string may not contain any other characters which
do not pertain to the name of the day of week, especially no leading
or trailing whitespace.

Note also that matching is performed in a case-insensitive manner (this may
depend on the "locale" setting on your current system, though!)

With "English" as the currently selected language (which is the default),
the following examples will all return the value "C<3>":

  $dow = Decode_Day_of_Week("w");
  $dow = Decode_Day_of_Week("Wed");
  $dow = Decode_Day_of_Week("wednes");
  $dow = Decode_Day_of_Week("Wednesday");

=item *

C<if ($lang = Decode_Language($string))>

This function takes a string as its argument, which should contain the
name of one of the languages supported by this package (B<IN THIS VERY
LANGUAGE ITSELF>), or any uniquely identifying abbreviation of the name
of a language (i.e., the first few letters), and returns its corresponding
internal number (1..13 in the original distribution) upon a successful match,
or "C<0>" otherwise (therefore, the return value can also be used as the
conditional expression in an "if" statement).

Note that the input string may not contain any other characters which do
not pertain to the name of a language, especially no leading or trailing
whitespace.

Note also that matching is performed in a case-insensitive manner (this may
depend on the "locale" setting on your current system, though!)

The original distribution supports the following thirteen languages:

            English                    ==>    1    (default)