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NAME

SYNOPSIS

#include <librock/sdncalh.h>

void
librock_SdnToGregorian(
        long int  sdn,
        int      *pYear,
        int      *pMonth,
        int      *pDay);

 /* Convert a SDN to a Gregorian calendar date.  If the input SDN is less
 * than 1, the three output values will all be set to zero, otherwise
 * *pYear will be >= -4714 and != 0; *pMonth will be in the range 1 to 12
 * inclusive; *pDay will be in the range 1 to 31 inclusive.
 */
long int
librock_GregorianToSdn(
        int inputYear,
        int inputMonth,
        int inputDay);

 /* Convert a Gregorian calendar date to a SDN.  Zero is returned when the
 * input date is detected as invalid or out of the supported range.  The
 * return value will be > 0 for all valid, supported dates, but there are
 * some invalid dates that will return a positive value.  To verify that a
 * date is valid, convert it to SDN and then back and compare with the
 * original.
 */

char *librock_MonthNameShort[13];
 /*
 * Convert a Gregorian month number (1 to 12) to the abbreviated (three
 * character) name of the Gregorian month (null terminated).  An index of
 * zero will return a zero length string.
 */

char *librock_MonthNameLong[13];
 /*
 * Convert a Gregorian month number (1 to 12) to the name of the Gregorian
 * month (null terminated).  An index of zero will return a zero length
 * string.
 */

DESCRIPTION

 /* This package defines a set of routines that convert calendar dates to
 * and from a serial day number (SDN).  The SDN is a serial numbering of
 * days where SDN 1 is November 25, 4714 BC in the Gregorian calendar and
 * SDN 2447893 is January 1, 1990.  This system of day numbering is
 * sometimes referred to as Julian days, but to avoid confusion with the
 * Julian calendar, it is referred to as serial day numbers here.  The term
 * Julian days is also used to mean the number of days since the beginning
 * of the current year.
 *
 * The SDN can be used as an intermediate step in converting from one
 * calendar system to another (such as Gregorian to Jewish).  It can also
 * be used for date computations such as easily comparing two dates,
 * determining the day of the week, finding the date of yesterday or
 * calculating the number of days between two dates.
 *
 * SDN values less than one are not supported.  If a conversion routine
 * returns an SDN of zero, this means that the date given is either invalid
 * or is outside the supported range for that calendar.
 *
 * At least some validity checks are performed on input dates.  For
 * example, a negative month number will result in the return of zero for
 * the SDN.  A returned SDN greater than one does not necessarily mean that
 * the input date was valid.  To determine if the date is valid, convert it
 * to SDN, and if the SDN is greater than zero, convert it back to a date
 * and compare to the original.  For example:
 */
     int y1, m1, d1;
     int y2, m2, d2;
     long int sdn;
     ...
     sdn = GregorianToSdn(y1, m1, d1);
     if (sdn > 0) {
         SdnToGregorian(sdn, &y2, &m2, &d2);
         if (y1 == y2 && m1 == m2 && d1 == d2) {
             ... date is valid ...
         }
     }

VALID RANGE

 *     4714 B.C. to at least 10000 A.D.
 *
 *     Although this software can handle dates all the way back to 4714
 *     B.C., such use may not be meaningful.  The Gregorian calendar was
 *     not instituted until October 15, 1582 (or October 5, 1582 in the
 *     Julian calendar).  Some countries did not accept it until much
 *     later.  For example, Britain converted in 1752, The USSR in 1918 and
 *     Greece in 1923.  Most European countries used the Julian calendar
 *     prior to the Gregorian.
 

CALENDAR OVERVIEW

 *
 *     The Gregorian calendar is a modified version of the Julian calendar.
 *     The only difference being the specification of leap years.  The
 *     Julian calendar specifies that every year that is a multiple of 4
 *     will be a leap year.  This leads to a year that is 365.25 days long,
 *     but the current accepted value for the tropical year is 365.242199
 *     days.
 *
 *     To correct this error in the length of the year and to bring the
 *     vernal equinox back to March 21, Pope Gregory XIII issued a papal
 *     bull declaring that Thursday October 4, 1582 would be followed by
 *     Friday October 15, 1582 and that centennial years would only be a
 *     leap year if they were a multiple of 400.  This shortened the year
 *     by 3 days per 400 years, giving a year of 365.2425 days.
 *
 *     Another recently proposed change in the leap year rule is to make
 *     years that are multiples of 4000 not a leap year, but this has never
 *     been officially accepted and this rule is not implemented in these
 *     algorithms.
 *

ALGORITHMS

 *
 *     The calculations are based on three different cycles: a 400 year
 *     cycle of leap years, a 4 year cycle of leap years and a 5 month
 *     cycle of month lengths.
 *
 *     The 5 month cycle is used to account for the varying lengths of
 *     months.  You will notice that the lengths alternate between 30
 *     and 31 days, except for three anomalies: both July and August
 *     have 31 days, both December and January have 31, and February
 *     is less than 30.  Starting with March, the lengths are in a
 *     cycle of 5 months (31, 30, 31, 30, 31):
 *
 *         Mar   31 days  \
 *         Apr   30 days   |
 *         May   31 days    > First cycle
 *         Jun   30 days   |
 *         Jul   31 days  /
 *
 *         Aug   31 days  \
 *         Sep   30 days   |
 *         Oct   31 days    > Second cycle
 *         Nov   30 days   |
 *         Dec   31 days  /
 *
 *         Jan   31 days  \
 *         Feb 28/9 days   |
 *                          > Third cycle (incomplete)
 *
 *     For this reason the calculations (internally) assume that the
 *     year starts with March 1.
 

TESTING

 *
 *     This algorithm has been tested from the year 4714 B.C. to 10000
 *     A.D.  The source code of the verification program is included in
 *     this package.
 

REFERENCES

 *
 *     Conversions Between Calendar Date and Julian Day Number by Robert J.
 *     Tantzen, Communications of the Association for Computing Machinery
 *     August 1963.  (Also published in Collected Algorithms from CACM,
 *     algorithm number 199).
 

USES

  // No external calls

LICENSE

  Copyright 1993-1995, Scott E. Lee, all rights reserved.
  Licensed under BSD-ish license, NO WARRANTY. Copies must retain this block.
  License text in <librock/license/sdncal.txt> librock_LIDESC_HC=553e181388455f0eef17ccd9e2a51c1f3ae8daf0

Source Code

This software is part of Librock

• Source code which is Open-source, Free (Libre) and free (no cost)
• Namespace limited. All compile-time macros and names visible at link time are prefixed by "librock_". Integrate into your project and expect no name collisions.
• Stable API means new functions get new names so that there is no risk that updating a source file will break existing applications.
• Highly portable, cross-platform, and multithread compatible. Works with many machines, compilers, and operating systems (gcc/MSVC/Windows/Linux/BSD).

Verbatim copying and distribution of this generated page is permitted in any medium provided that no changes are made.
(The source of this manual page may be covered by a more permissive license which allows modifications.)

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