112 lines
2.4 KiB
Groff
112 lines
2.4 KiB
Groff
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.TH caldate_mjd 3
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.SH NAME
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caldate_mjd \- manipulate calendar dates
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.SH SYNTAX
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.B #include <caldate.h>
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void \fBcaldate_frommjd\fP(&\fIcd\fR,\fIday\fR,\fIweekday\fR,\fIyearday\fR);
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.br
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long \fBcaldate_mjd\fP(&\fIcd\fR);
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.br
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void \fBcaldate_normalize\fP(&\fIcd\fR);
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void \fBcaldate_easter\fP(&\fIcd\fR);
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struct caldate \fIcd\fR;
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.br
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long \fIday\fR;
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.br
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int *\fIweekday\fR;
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.br
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int *\fIyearday\fR;
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.SH "BASIC CONVERSIONS"
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Every calendar date has a
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.I modified Julian day number\fR.
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The day number increases by 1 every day.
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Day number 0 is 17 November 1858.
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Day number 51604 is 1 March 2000.
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.B caldate_frommjd
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puts into
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.I cd
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the date corresponding to the modified Julian day number
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.IR day .
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.B caldate_frommjd
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also computes the day of the week (0 through 6)
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and the day of the year (0 through 365).
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It stores the day of the week in
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.B *\fIweekday
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if
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.I weekday
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is nonzero.
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It stores the day of the year in
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.B *\fIyearday
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if
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.I yearday
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is nonzero.
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.B caldate_mjd
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returns the modified Julian day number corresponding to the date in
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.IR cd .
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.B caldate_mjd
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accepts days outside the range 1 to 31,
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referring to days before the beginning or after the end of the month.
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It also accepts months outside the range 1 to 12,
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referring to months before the beginning or after the end of the year.
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.B caldate_normalize
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calls
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.B caldate_frommjd
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with the result of
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.BR caldate_mjd .
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This means that it accepts out-of-range months and out-of-range days in
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.IR cd ,
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and puts a valid calendar date back into
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.IR cd .
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.SH "OTHER FUNCTIONS"
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.B caldate_easter
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reads the year from
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.I cd
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and then changes
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.I cd
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to the date of Easter in the same year.
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.SH LIMITATIONS
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The
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.B caldate
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routines currently support the Gregorian calendar,
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which was defined in 1582 and adopted at different times
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in different countries.
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For earlier dates the
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.B caldate
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routines work with
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``virtual Gregorian,''
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defined mathematically by the 400-year Gregorian cycle
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for years before 1582.
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The Julian calendar is not supported.
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The Gregorian calendar will be replaced by a new calendar
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within a few thousand years.
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The
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.B caldate_frommjd
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and
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.B caldate_mjd
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routines will be upgraded accordingly.
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The current
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.B caldate_frommjd
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and
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.B caldate_mjd
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routines are not useful for dates far in the future.
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Day numbers will overflow a 32-bit
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.B long
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sometime after the year 5000000;
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all systems should upgrade to 64-bit
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.BR long s
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before then.
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.B caldate_mjd
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does not check for overflow.
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.SH "SEE ALSO"
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caldate(3)
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