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264 lines
8.0 KiB
C
264 lines
8.0 KiB
C
/* $selId: julian.c,v 2.0 1995/10/24 01:13:06 lees Exp $
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* Copyright 1993-1995, Scott E. Lee, all rights reserved.
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* Permission granted to use, copy, modify, distribute and sell so long as
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* the above copyright and this permission statement are retained in all
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* copies. THERE IS NO WARRANTY - USE AT YOUR OWN RISK.
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*/
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/**************************************************************************
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*
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* These are the externally visible components of this file:
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*
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* void
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* SdnToJulian(
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* long int sdn,
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* int *pYear,
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* int *pMonth,
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* int *pDay);
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*
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* Convert a SDN to a Julian calendar date. If the input SDN is less than
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* 1, the three output values will all be set to zero, otherwise *pYear
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* will be >= -4713 and != 0; *pMonth will be in the range 1 to 12
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* inclusive; *pDay will be in the range 1 to 31 inclusive.
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*
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* long int
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* JulianToSdn(
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* int inputYear,
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* int inputMonth,
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* int inputDay);
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*
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* Convert a Julian calendar date to a SDN. Zero is returned when the
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* input date is detected as invalid or out of the supported range. The
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* return value will be > 0 for all valid, supported dates, but there are
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* some invalid dates that will return a positive value. To verify that a
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* date is valid, convert it to SDN and then back and compare with the
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* original.
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*
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* VALID RANGE
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*
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* 4713 B.C. to at least 10000 A.D.
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*
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* Although this software can handle dates all the way back to 4713
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* B.C., such use may not be meaningful. The calendar was created in
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* 46 B.C., but the details did not stabilize until at least 8 A.D.,
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* and perhaps as late at the 4th century. Also, the beginning of a
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* year varied from one culture to another - not all accepted January
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* as the first month.
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*
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* CALENDAR OVERVIEW
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*
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* Julias Ceasar created the calendar in 46 B.C. as a modified form of
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* the old Roman republican calendar which was based on lunar cycles.
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* The new Julian calendar set fixed lengths for the months, abandoning
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* the lunar cycle. It also specified that there would be exactly 12
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* months per year and 365.25 days per year with every 4th year being a
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* leap year.
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*
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* Note that the current accepted value for the tropical year is
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* 365.242199 days, not 365.25. This lead to an 11 day shift in the
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* calendar with respect to the seasons by the 16th century when the
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* Gregorian calendar was created to replace the Julian calendar.
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*
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* The difference between the Julian and today's Gregorian calendar is
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* that the Gregorian does not make centennial years leap years unless
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* they are a multiple of 400, which leads to a year of 365.2425 days.
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* In other words, in the Gregorian calendar, 1700, 1800 and 1900 are
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* not leap years, but 2000 is. All centennial years are leap years in
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* the Julian calendar.
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*
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* The details are unknown, but the lengths of the months were adjusted
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* until they finally stablized in 8 A.D. with their current lengths:
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*
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* January 31
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* February 28/29
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* March 31
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* April 30
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* May 31
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* June 30
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* Quintilis/July 31
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* Sextilis/August 31
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* September 30
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* October 31
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* November 30
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* December 31
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*
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* In the early days of the calendar, the days of the month were not
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* numbered as we do today. The numbers ran backwards (decreasing) and
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* were counted from the Ides (15th of the month - which in the old
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* Roman republican lunar calendar would have been the full moon) or
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* from the Nonae (9th day before the Ides) or from the beginning of
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* the next month.
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*
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* In the early years, the beginning of the year varied, sometimes
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* based on the ascension of rulers. It was not always the first of
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* January.
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*
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* Also, today's epoch, 1 A.D. or the birth of Jesus Christ, did not
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* come into use until several centuries later when Christianity became
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* a dominant religion.
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*
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* ALGORITHMS
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*
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* The calculations are based on two different cycles: a 4 year cycle
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* of leap years and a 5 month cycle of month lengths.
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*
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* The 5 month cycle is used to account for the varying lengths of
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* months. You will notice that the lengths alternate between 30 and
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* 31 days, except for three anomalies: both July and August have 31
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* days, both December and January have 31, and February is less than
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* 30. Starting with March, the lengths are in a cycle of 5 months
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* (31, 30, 31, 30, 31):
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*
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* Mar 31 days \
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* Apr 30 days |
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* May 31 days > First cycle
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* Jun 30 days |
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* Jul 31 days /
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*
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* Aug 31 days \
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* Sep 30 days |
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* Oct 31 days > Second cycle
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* Nov 30 days |
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* Dec 31 days /
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*
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* Jan 31 days \
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* Feb 28/9 days |
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* > Third cycle (incomplete)
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*
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* For this reason the calculations (internally) assume that the year
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* starts with March 1.
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*
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* TESTING
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*
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* This algorithm has been tested from the year 4713 B.C. to 10000 A.D.
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* The source code of the verification program is included in this
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* package.
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*
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* REFERENCES
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*
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* Conversions Between Calendar Date and Julian Day Number by Robert J.
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* Tantzen, Communications of the Association for Computing Machinery
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* August 1963. (Also published in Collected Algorithms from CACM,
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* algorithm number 199). [Note: the published algorithm is for the
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* Gregorian calendar, but was adjusted to use the Julian calendar's
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* simpler leap year rule.]
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*
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**************************************************************************/
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#include "sdncal.h"
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#include <limits.h>
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#define JULIAN_SDN_OFFSET 32083
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#define DAYS_PER_5_MONTHS 153
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#define DAYS_PER_4_YEARS 1461
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void SdnToJulian(
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long int sdn,
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int *pYear,
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int *pMonth,
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int *pDay)
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{
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int year;
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int month;
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int day;
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long int temp;
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int dayOfYear;
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if (sdn <= 0) {
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goto fail;
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}
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/* Check for overflow */
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if (sdn > (LONG_MAX - JULIAN_SDN_OFFSET * 4 + 1) / 4 || sdn < LONG_MIN / 4) {
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goto fail;
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}
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temp = sdn * 4 + (JULIAN_SDN_OFFSET * 4 - 1);
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/* Calculate the year and day of year (1 <= dayOfYear <= 366). */
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{
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long yearl = temp / DAYS_PER_4_YEARS;
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if (yearl > INT_MAX || yearl < INT_MIN) {
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goto fail;
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}
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year = (int) yearl;
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}
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dayOfYear = (temp % DAYS_PER_4_YEARS) / 4 + 1;
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/* Calculate the month and day of month. */
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temp = dayOfYear * 5 - 3;
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month = temp / DAYS_PER_5_MONTHS;
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day = (temp % DAYS_PER_5_MONTHS) / 5 + 1;
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/* Convert to the normal beginning of the year. */
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if (month < 10) {
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month += 3;
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} else {
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year += 1;
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month -= 9;
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}
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/* Adjust to the B.C./A.D. type numbering. */
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year -= 4800;
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if (year <= 0)
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year--;
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*pYear = year;
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*pMonth = month;
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*pDay = day;
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return;
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fail:
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*pYear = 0;
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*pMonth = 0;
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*pDay = 0;
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}
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long int JulianToSdn(
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int inputYear,
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int inputMonth,
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int inputDay)
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{
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int year;
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int month;
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/* check for invalid dates */
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if (inputYear == 0 || inputYear < -4713 ||
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inputMonth <= 0 || inputMonth > 12 ||
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inputDay <= 0 || inputDay > 31) {
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return (0);
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}
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/* check for dates before SDN 1 (Jan 2, 4713 B.C.) */
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if (inputYear == -4713) {
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if (inputMonth == 1 && inputDay == 1) {
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return (0);
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}
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}
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/* Make year always a positive number. */
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if (inputYear < 0) {
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year = inputYear + 4801;
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} else {
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year = inputYear + 4800;
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}
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/* Adjust the start of the year. */
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if (inputMonth > 2) {
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month = inputMonth - 3;
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} else {
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month = inputMonth + 9;
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year--;
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}
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return ((year * DAYS_PER_4_YEARS) / 4
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+ (month * DAYS_PER_5_MONTHS + 2) / 5
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+ inputDay
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- JULIAN_SDN_OFFSET);
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}
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/*
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* Local variables:
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* tab-width: 4
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* c-basic-offset: 4
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* End:
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* vim600: sw=4 ts=4 fdm=marker
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* vim<600: sw=4 ts=4
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*/
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