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 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
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
<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
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.