usr/src/sys/tahoe/include/clock.h

/*      clock.h 4.5     81/02/23        */

#define SECDAY          ((unsigned)(24*60*60))          /* seconds per day */
#define SECYR           ((unsigned)(365*SECDAY))        /* per common year */

#define YRREF           1970
#define LEAPYEAR(year)  ((year)%4==0)   /* good till time becomes negative */

/*
 * Software clock is software interrupt level 8
 */
#define setsoftclock()  mtpr(0x8, SIRR)