adjust grammar in a few man pages: s/was/were/
[dragonfly.git] / lib / libcalendar / calendar.3
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1.\" Copyright (c) 1997 Wolfgang Helbig
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25.\" $FreeBSD: src/lib/libcalendar/calendar.3,v 1.6.2.5 2001/12/17 10:08:28 ru Exp $
26.\"
27.Dd November 29, 1997
28.Dt CALENDAR 3
29.Os
30.Sh NAME
31.Nm easterg ,
32.Nm easterog ,
33.Nm easteroj ,
34.Nm gdate ,
35.Nm jdate ,
36.Nm ndaysg ,
37.Nm ndaysj ,
38.Nm week ,
39.Nm weekday
40.Nd Calendar arithmetic for the Christian era
41.Sh LIBRARY
42.Lb libcalendar
43.Sh SYNOPSIS
44.In calendar.h
45.Ft struct date *
46.Fn easterg "int year" "struct date *dt"
47.Ft struct date *
48.Fn easterog "int year" "struct date *dt"
49.Ft struct date *
50.Fn easteroj "int year" "struct date *dt"
51.Ft struct date *
52.Fn gdate "int nd" "struct date *dt"
53.Ft struct date *
54.Fn jdate "int nd" "struct date *dt"
55.Ft int
56.Fn ndaysg "struct date *dt"
57.Ft int
58.Fn ndaysj "struct date *dt"
59.Ft int
60.Fn week "int nd" "int *year"
61.Ft int
62.Fn weekday "int nd"
63.Sh DESCRIPTION
64These functions provide calendar arithmetic for a large range of years,
65starting at March 1st, year zero (i. e. 1 B.C.) and ending way beyond
66year 100000.
67.Pp
68Programs should be linked with
69.Fl lcalendar .
70.Pp
71The functions
72.Fn easterg ,
73.Fn easterog
74and
75.Fn easteroj
76store the date of Easter Sunday into the structure pointed at by
77.Fa dt
78and return a pointer to this structure.
79The function
80.Fn easterg
81assumes Gregorian Calendar (adopted by most western churches after 1582) and
82the functions
83.Fn easterog
84and
85.Fn easteroj
86compute the date of Easter Sunday according to the orthodox rules
87(Western churches before 1582, Greek and Russian Orthodox Church
88until today).
89The result returned by
90.Fn easterog
91is the date in Gregorian Calendar, whereas
92.Fn easteroj
93returns the date in Julian Calendar.
94.Pp
95The functions
96.Fn gdate ,
97.Fn jdate ,
98.Fn ndaysg
99and
100.Fn ndaysj
101provide conversions between the common "year, month, day" notation
102of a date and the "number of days" representation, which is better suited
103for calculations.
104The days are numbered from March 1st year 1 B.C., starting
105with zero, so the number of a day gives the number of days since March 1st,
106year 1 B.C. The conversions work for nonnegative day numbers only.
107.Pp
108The
109.Fn gdate
110and
111.Fn jdate
112functions
113store the date corresponding to the day number
114.Fa nd
115into the structure pointed at by
116.Fa dt
117and return a pointer to this structure.
118.Pp
119The
120.Fn ndaysg
121and
122.Fn ndaysj
123functions
124return the day number of the date pointed at by
125.Fa dt .
126.Pp
127The
128.Fn gdate
129and
130.Fn ndaysg
131functions
132assume Gregorian Calendar after October 4, 1582 and Julian Calendar before,
133whereas
134.Fn jdate
135and
136.Fn ndaysj
137assume Julian Calendar throughout.
138.Pp
139The two calendars differ by the definition of the leap year.
140The
141Julian Calendar says every year that is a multiple of four is a
142leap year.
143The Gregorian Calendar excludes years that are multiples of
144100 and not multiples of 400.
145This means the years 1700, 1800, 1900, 2100 are not leap years
146and the year 2000 is
147a leap year.
148The new rules were inaugurated on October 4, 1582 by deleting ten
149days following this date.
150Most catholic countries adopted the new
151calendar by the end of the 16th century, whereas others stayed with
152the Julian Calendar until the 20th century.
153The United Kingdom and
154their colonies switched on September 2, 1752. They already had to
155delete 11 days.
156.Pp
157The function
158.Fn week
159returns the number of the week which contains the day numbered
160.Fa nd .
161The argument
162.Fa *year
163is set with the year that contains (the greater part of) the week.
164The weeks are numbered per year starting with week 1, which is the
165first week in a year that includes more than three days of the year.
166Weeks start on Monday.
167This function is defined for Gregorian Calendar only.
168.Pp
169The function
170.Fn weekday
171returns the weekday (Mo = 0 .. Su = 6) of the day numbered
172.Fa nd .
173.Pp
174The structure
175.Fa date
176is defined in
44cb301e 177.In calendar.h .
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178It contains these fields:
179.Bd -literal -offset indent
180int y; /\(** year (0000 - ????) \(**/
181int m; /\(** month (1 - 12) \(**/
182int d; /\(** day of month (1 - 31) \(**/
183.Ed
184.Pp
185The year zero is written as "1 B.C." by historians and "0" by astronomers
186and in this library.
187.Sh SEE ALSO
188.Xr ncal 1 ,
189.Xr strftime 3
190.Sh STANDARDS
191The week number conforms to ISO 8601: 1988.
192.Sh HISTORY
193The
194.Nm calendar
195library first appeared in
196.Fx 3.0 .
197.Sh AUTHORS
97a96273 198This manual page and the library were written by
98b3d9ad 199.An Wolfgang Helbig Aq Mt helbig@FreeBSD.org .
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200.Sh BUGS
201The library was coded with great care so there are no bugs left.