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Editing: calendar.3
.\" Copyright (c) 1997 Wolfgang Helbig .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" $FreeBSD$ .\" .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 Mt helbig@FreeBSD.org . .Sh BUGS The library was coded with great care so there are no bugs left.
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