calendar(3)

NAME

easterg, easterog, easteroj, gdate, jdate, ndaysg, ndaysj,

week, weekday
- Calendar arithmetic for the Christian era

LIBRARY

library ``libcalendar''

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 func

tions
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() pro

vide conversions
between the common "year, month, day" notation of a date and

the "number
of days" representation, which is better suited for calcula

tions. 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 correspond

ing 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 October 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 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.

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 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.

BSD November 29, 1997

BSD