+.Ar yymmddhhmm ,
+where the year, month, and day may be defaulted
+to the current system values. The first form brings the system down in
+.Ar number
+minutes and the second at the absolute time specified.
+.It Ar warning-message
+Any other arguments comprise the warning message that is broadcast
+to users currently logged into the system.
+.It Fl
+If
+.Ql Fl
+is supplied as an option, the warning message is read from the standard
+input.
+.El
+.Pp
+At intervals, becoming more frequent as apocalypse approaches
+and starting at ten hours before shutdown, warning messages are displayed
+on the terminals of all users logged in. Five minutes before
+shutdown, or immediately if shutdown is in less than 5 minutes,
+logins are disabled by creating
+.Pa /etc/nologin
+and copying the
+warning message there. If this file exists when a user attempts to
+log in,
+.Xr login 1
+prints its contents and exits. The file is
+removed just before
+.Nm shutdown
+exits.
+.Pp
+At shutdown time a message is written in the system log, containing the
+time of shutdown, who initiated the shutdown and the reason.
+A terminate
+signal is then sent to
+.Xr init
+to bring the system down to single-user state (depending on above
+options).