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NAME

SYNOPSIS

#include <librock/sdncalh.h>

void
librock_SdnToJulian(
    long int  sdn,
    int      *pYear,
    int      *pMonth,
    int      *pDay);
 /*
 * Convert a SDN to a Julian calendar date.  If the input SDN is less than
 * 1, the three output values will all be set to zero, otherwise *pYear
 * will be >= -4713 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_JulianToSdn(
    int inputYear,
    int inputMonth,
    int inputDay);
 /*
 * Convert a Julian 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.
 */

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

 /*
 *     4713 B.C. to at least 10000 A.D.
 *
 *     Although this software can handle dates all the way back to 4713
 *     B.C., such use may not be meaningful.  The calendar was created in
 *     46 B.C., but the details did not stabilize until at least 8 A.D.,
 *     and perhaps as late at the 4th century.  Also, the beginning of a
 *     year varied from one culture to another - not all accepted January
 *     as the first month.
 */

CALENDAR OVERVIEW

 /*
 *     Julias Ceasar created the calendar in 46 B.C. as a modified form of
 *     the old Roman republican calendar which was based on lunar cycles.
 *     The new Julian calendar set fixed lengths for the months, abandoning
 *     the lunar cycle.  It also specified that there would be exactly 12
 *     months per year and 365.25 days per year with every 4th year being a
 *     leap year.
 *
 *     Note that the current accepted value for the tropical year is
 *     365.242199 days, not 365.25.  This lead to an 11 day shift in the
 *     calendar with respect to the seasons by the 16th century when the
 *     Gregorian calendar was created to replace the Julian calendar.
 *
 *     The difference between the Julian and today's Gregorian calendar is
 *     that the Gregorian does not make centennial years leap years unless
 *     they are a multiple of 400, which leads to a year of 365.2425 days.
 *     In other words, in the Gregorian calendar, 1700, 1800 and 1900 are
 *     not leap years, but 2000 is.  All centennial years are leap years in
 *     the Julian calendar.
 *
 *     The details are unknown, but the lengths of the months were adjusted
 *     until they finally stablized in 8 A.D. with their current lengths:
 *
 *         January          31
 *         February         28/29
 *         March            31
 *         April            30
 *         May              31
 *         June             30
 *         Quintilis/July   31
 *         Sextilis/August  31
 *         September        30
 *         October          31
 *         November         30
 *         December         31
 *
 *     In the early days of the calendar, the days of the month were not
 *     numbered as we do today.  The numbers ran backwards (decreasing) and
 *     were counted from the Ides (15th of the month - which in the old
 *     Roman republican lunar calendar would have been the full moon) or
 *     from the Nonae (9th day before the Ides) or from the beginning of
 *     the next month.
 *
 *     In the early years, the beginning of the year varied, sometimes
 *     based on the ascension of rulers.  It was not always the first of
 *     January.
 *
 *     Also, today's epoch, 1 A.D. or the birth of Jesus Christ, did not
 *     come into use until several centuries later when Christianity became
 *     a dominant religion.
 */

ALGORITHMS

 /*
 *     The calculations are based on two different cycles: 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 4713 B.C. to 10000 A.D.
 *     The source code of the verification program is included in sdncal.
 */

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).  [Note: the published algorithm is for the
 *     Gregorian calendar, but was adjusted to use the Julian calendar's
 *     simpler leap year rule.]
 */

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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