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40 .Nd Calendar arithmetic for the Christian era
46 .Fn easterg "int year" "struct date *dt"
48 .Fn easterog "int year" "struct date *dt"
50 .Fn easteroj "int year" "struct date *dt"
52 .Fn gdate "int nd" "struct date *dt"
54 .Fn jdate "int nd" "struct date *dt"
56 .Fn ndaysg "struct date *dt"
58 .Fn ndaysj "struct date *dt"
60 .Fn week "int nd" "int *year"
64 These functions provide calendar arithmetic for a large range of years,
65 starting at March 1st, year zero (i.e., 1 B.C.) and ending way beyond
68 Programs should be linked with
76 store the date of Easter Sunday into the structure pointed at by
78 and return a pointer to this structure.
81 assumes Gregorian Calendar (adopted by most western churches after 1582) and
86 compute the date of Easter Sunday according to the orthodox rules
87 (Western churches before 1582, Greek and Russian Orthodox Church
89 The result returned by
91 is the date in Gregorian Calendar, whereas
93 returns the date in Julian Calendar.
101 provide conversions between the common "year, month, day" notation
102 of a date and the "number of days" representation, which is better suited
104 The days are numbered from March 1st year 1 B.C., starting
105 with zero, so the number of a day gives the number of days since March 1st,
107 The conversions work for nonnegative day numbers only.
114 store the date corresponding to the day number
116 into the structure pointed at by
118 and return a pointer to this structure.
125 return the day number of the date pointed at by
133 assume Gregorian Calendar after October 4, 1582 and Julian Calendar before,
138 assume Julian Calendar throughout.
140 The two calendars differ by the definition of the leap year.
142 Julian Calendar says every year that is a multiple of four is a
144 The Gregorian Calendar excludes years that are multiples of
145 100 and not multiples of 400.
146 This means the years 1700, 1800, 1900, 2100 are not leap years
149 The new rules were inaugurated on October 4, 1582 by deleting ten
150 days following this date.
151 Most catholic countries adopted the new
152 calendar by the end of the 16th century, whereas others stayed with
153 the Julian Calendar until the 20th century.
154 The United Kingdom and
155 their colonies switched on September 2, 1752.
161 returns the number of the week which contains the day numbered
165 is set with the year that contains (the greater part of) the week.
166 The weeks are numbered per year starting with week 1, which is the
167 first week in a year that includes more than three days of the year.
168 Weeks start on Monday.
169 This function is defined for Gregorian Calendar only.
173 returns the weekday (Mo = 0 ..\& Su = 6) of the day numbered
180 It contains these fields:
181 .Bd -literal -offset indent
182 int y; /\(** year (0000 - ????) \(**/
183 int m; /\(** month (1 - 12) \(**/
184 int d; /\(** day of month (1 - 31) \(**/
187 The year zero is written as "1 B.C." by historians and "0" by astronomers
193 The week number conforms to ISO 8601: 1988.
197 library first appeared in
200 This manual page and the library was written by
201 .An Wolfgang Helbig Aq helbig@FreeBSD.org .
203 The library was coded with great care so there are no bugs left.