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easterg easterog easteroj gdate jdate ndaysg ndaysj week weekday - Calendar arithmetic for the Christian era
Lb libcalendar
Programs should be linked with -lcalendar
The functions
easterg (,);
easterog ();
and
easteroj ();
store the date of Easter Sunday into the structure pointed at by
Fa 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
Fa nd
into the structure pointed at by
Fa dt
and return a pointer to this structure.
The
ndaysg ();
and
ndaysj ();
functions
return the day number of the date pointed at by
Fa 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
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.
The function
weekday ();
returns the weekday (Mo = 0 .. Su = 6) of the day numbered
Fa nd .
The structure Fa date is defined in 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.
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