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caldate_mjd(3)        DragonFly Library Functions Manual        caldate_mjd(3)

NAME
       caldate_mjd - manipulate calendar dates

SYNTAX
       #include <caldate.h>

       void caldate_frommjd(&cd,day,weekday,yearday);
       long caldate_mjd(&cd);
       void caldate_normalize(&cd);

       void caldate_easter(&cd);

       struct caldate cd;
       long day;
       int *weekday;
       int *yearday;

BASIC CONVERSIONS
       Every calendar date has a modified Julian day number.  The day number
       increases by 1 every day.  Day number 0 is 17 November 1858.  Day
       number 51604 is 1 March 2000.

       caldate_frommjd puts into cd the date corresponding to the modified
       Julian day number day.

       caldate_frommjd also computes the day of the week (0 through 6) and the
       day of the year (0 through 365).  It stores the day of the week in
       *weekday if weekday is nonzero.  It stores the day of the year in
       *yearday if yearday is nonzero.

       caldate_mjd returns the modified Julian day number corresponding to the
       date in cd.

       caldate_mjd accepts days outside the range 1 to 31, referring to days
       before the beginning or after the end of the month.  It also accepts
       months outside the range 1 to 12, referring to months before the
       beginning or after the end of the year.

       caldate_normalize calls caldate_frommjd with the result of caldate_mjd.
       This means that it accepts out-of-range months and out-of-range days in
       cd, and puts a valid calendar date back into cd.

OTHER FUNCTIONS
       caldate_easter reads the year from cd and then changes cd to the date
       of Easter in the same year.

LIMITATIONS
       The caldate routines currently support the Gregorian calendar, which
       was defined in 1582 and adopted at different times in different
       countries.  For earlier dates the caldate routines work with ``virtual
       Gregorian,'' defined mathematically by the 400-year Gregorian cycle for
       years before 1582.  The Julian calendar is not supported.

       The Gregorian calendar will be replaced by a new calendar within a few
       thousand years.  The caldate_frommjd and caldate_mjd routines will be
       upgraded accordingly.  The current caldate_frommjd and caldate_mjd
       routines are not useful for dates far in the future.

       Day numbers will overflow a 32-bit long sometime after the year
       5000000; all systems should upgrade to 64-bit longs before then.
       caldate_mjd does not check for overflow.

SEE ALSO
       caldate(3)

                                                                caldate_mjd(3)