.\" Copyright (c) 1997 Wolfgang Helbig
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.\" $FreeBSD: src/lib/libcalendar/calendar.3,v 1.6.2.5 2001/12/17 10:08:28 ru Exp $
.\" $DragonFly: src/lib/libcalendar/calendar.3,v 1.3 2006/05/26 19:39:38 swildner Exp $
.\"
.Dd November 29, 1997
.Dt CALENDAR 3
.Os
.Sh NAME
.Nm easterg ,
.Nm easterog ,
.Nm easteroj ,
.Nm gdate ,
.Nm jdate ,
.Nm ndaysg ,
.Nm ndaysj ,
.Nm week ,
.Nm weekday
.Nd Calendar arithmetic for the Christian era
.Sh LIBRARY
.Lb libcalendar
.Sh SYNOPSIS
.In calendar.h
.Ft struct date *
.Fn easterg "int year" "struct date *dt"
.Ft struct date *
.Fn easterog "int year" "struct date *dt"
.Ft struct date *
.Fn easteroj "int year" "struct date *dt"
.Ft struct date *
.Fn gdate "int nd" "struct date *dt"
.Ft struct date *
.Fn jdate "int nd" "struct date *dt"
.Ft int
.Fn ndaysg "struct date *dt"
.Ft int
.Fn ndaysj "struct date *dt"
.Ft int
.Fn week "int nd" "int *year"
.Ft int
.Fn weekday "int nd"
.Sh 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.
.Pp
Programs should be linked with
.Fl lcalendar .
.Pp
The functions
.Fn easterg ,
.Fn easterog
and
.Fn easteroj
store the date of Easter Sunday into the structure pointed at by
.Fa dt
and return a pointer to this structure.
The function
.Fn easterg
assumes Gregorian Calendar (adopted by most western churches after 1582) and
the functions
.Fn easterog
and
.Fn 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
.Fn easterog
is the date in Gregorian Calendar, whereas
.Fn easteroj
returns the date in Julian Calendar.
.Pp
The functions
.Fn gdate ,
.Fn jdate ,
.Fn ndaysg
and
.Fn 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.
.Pp
The
.Fn gdate
and
.Fn jdate
functions
store the date corresponding to the day number
.Fa nd
into the structure pointed at by
.Fa dt
and return a pointer to this structure.
.Pp
The
.Fn ndaysg
and
.Fn ndaysj
functions
return the day number of the date pointed at by
.Fa dt .
.Pp
The
.Fn gdate
and
.Fn ndaysg
functions
assume Gregorian Calendar after October 4, 1582 and Julian Calendar before,
whereas
.Fn jdate
and
.Fn ndaysj
assume Julian Calendar throughout.
.Pp
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 multiples 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 century, whereas others stayed with
the Julian Calendar until the 20th century.
The United Kingdom and
their colonies switched on September 2, 1752. They already had to
delete 11 days.
.Pp
The function
.Fn week
returns the number of the week which contains the day numbered
.Fa nd .
The argument
.Fa *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.
.Pp
The function
.Fn weekday
returns the weekday (Mo = 0 .. Su = 6) of the day numbered
.Fa nd .
.Pp
The structure
.Fa date
is defined in
.In calendar.h .
It contains these fields:
.Bd -literal -offset indent
int y;          /\(** year (0000 - ????) \(**/
int m;          /\(** month (1 - 12) \(**/
int d;          /\(** day of month (1 - 31) \(**/
.Ed
.Pp
The year zero is written as "1 B.C." by historians and "0" by astronomers
and in this library.
.Sh SEE ALSO
.Xr ncal 1 ,
.Xr strftime 3
.Sh STANDARDS
The week number conforms to ISO 8601: 1988.
.Sh HISTORY
The
.Nm calendar
library first appeared in
.Fx 3.0 .
.Sh AUTHORS
This manual page and the library was written by
.An Wolfgang Helbig Aq helbig@FreeBSD.org .
.Sh BUGS
The library was coded with great care so there are no bugs left.