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CALENDAR(3)	       FreeBSD Library Functions Manual		   CALENDAR(3)

NAME
     easterg, easterog,	easteroj, gdate, jdate,	ndaysg,	ndaysj,	week, weekday
     --	Calendar arithmetic for	the Christian era

LIBRARY
     Calendar Arithmetic Library (libcalendar, -lcalendar)

SYNOPSIS
     #include <calendar.h>

     struct date *
     easterg(int year, struct date *dt);

     struct date *
     easterog(int year,	struct date *dt);

     struct date *
     easteroj(int year,	struct date *dt);

     struct date *
     gdate(int nd, struct date *dt);

     struct date *
     jdate(int nd, struct date *dt);

     int
     ndaysg(struct date	*dt);

     int
     ndaysj(struct date	*dt);

     int
     week(int nd, int *year);

     int
     weekday(int nd);

DESCRIPTION
     These functions provide calendar arithmetic for a large range of years,
     starting at March 1st, year zero (i.e., 1 B.C.) and ending	way beyond
     year 100000.

     Programs should be	linked with -lcalendar.

     The functions easterg(), easterog() and easteroj()	store the date of
     Easter Sunday into	the structure pointed at by dt and return a pointer to
     this structure.  The function easterg() assumes Gregorian Calendar
     (adopted by most western churches after 1582) and the functions
     easterog()	and easteroj() compute the date	of Easter Sunday according to
     the orthodox rules	(Western churches before 1582, Greek and Russian
     Orthodox Church until today).  The	result returned	by easterog() is the
     date in Gregorian Calendar, whereas easteroj() returns the	date in	Julian
     Calendar.

     The functions gdate(), jdate(), ndaysg() and ndaysj() provide conversions
     between the common	"year, month, day" notation of a date and the "number
     of	days" representation, which is better suited for calculations.	The
     days are numbered from March 1st year 1 B.C., starting with zero, so the
     number of a day gives the number of days since March 1st, year 1 B.C.
     The conversions work for nonnegative day numbers only.

     The gdate() and jdate() functions store the date corresponding to the day
     number nd into the	structure pointed at by	dt and return a	pointer	to
     this structure.

     The ndaysg() and ndaysj() functions return	the day	number of the date
     pointed at	by dt.

     The gdate() and ndaysg() functions	assume Gregorian Calendar after	Octo-
     ber 4, 1582 and Julian Calendar before, whereas jdate() and ndaysj()
     assume Julian Calendar throughout.

     The two calendars differ by the definition	of the leap year.  The Julian
     Calendar says every year that is a	multiple of four is a leap year.  The
     Gregorian Calendar	excludes years that are	multiples of 100 and not mul-
     tiples of 400.  This means	the years 1700,	1800, 1900, 2100 are not leap
     years and the year	2000 is	a leap year.  The new rules were inaugurated
     on	October	4, 1582	by deleting ten	days following this date.  Most
     catholic countries	adopted	the new	calendar by the	end of the 16th	cen-
     tury, whereas others stayed with the Julian Calendar until	the 20th cen-
     tury.  The	United Kingdom and their colonies switched on September	2,
     1752.  They already had to	delete 11 days.

     The function week() returns the number of the week	which contains the day
     numbered nd.  The argument	*year is set with the year that	contains (the
     greater part of) the week.	 The weeks are numbered	per year starting with
     week 1, which is the first	week in	a year that includes more than three
     days of the year.	Weeks start on Monday.	This function is defined for
     Gregorian Calendar	only.

     The function weekday() returns the	weekday	(Mo = 0	.. Su =	6) of the day
     numbered nd.

     The structure date	is defined in <calendar.h>.  It	contains these fields:

	   int y;	   /* year (0000 - ????) */
	   int m;	   /* month (1 - 12) */
	   int d;	   /* day of month (1 -	31) */

     The year zero is written as "1 B.C." by historians	and "0"	by astronomers
     and in this library.

SEE ALSO
     ncal(1), strftime(3)

STANDARDS
     The week number conforms to ISO 8601: 1988.

HISTORY
     The calendar library first	appeared in FreeBSD 3.0.

AUTHORS
     This manual page and the library was written by Wolfgang Helbig
     <helbig@FreeBSD.org>.

BUGS
     The library was coded with	great care so there are	no bugs	left.

FreeBSD	9.3		       November	29, 1997		   FreeBSD 9.3

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | SEE ALSO | STANDARDS | HISTORY | AUTHORS | BUGS

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