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266 lines
7.3 KiB
C
266 lines
7.3 KiB
C
/* $selId: gregor.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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* SdnToGregorian(
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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 Gregorian calendar date. If the input SDN is less
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* than 1, the three output values will all be set to zero, otherwise
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* *pYear will be >= -4714 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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* GregorianToSdn(
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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 Gregorian 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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* char *MonthNameShort[13];
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*
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* Convert a Gregorian month number (1 to 12) to the abbreviated (three
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* character) name of the Gregorian month (null terminated). An index of
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* zero will return a zero length string.
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*
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* char *MonthNameLong[13];
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*
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* Convert a Gregorian month number (1 to 12) to the name of the Gregorian
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* month (null terminated). An index of zero will return a zero length
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* string.
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*
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* VALID RANGE
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*
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* 4714 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 4714
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* B.C., such use may not be meaningful. The Gregorian calendar was
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* not instituted until October 15, 1582 (or October 5, 1582 in the
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* Julian calendar). Some countries did not accept it until much
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* later. For example, Britain converted in 1752, The USSR in 1918 and
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* Greece in 1923. Most European countries used the Julian calendar
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* prior to the Gregorian.
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*
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* CALENDAR OVERVIEW
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*
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* The Gregorian calendar is a modified version of the Julian calendar.
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* The only difference being the specification of leap years. The
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* Julian calendar specifies that every year that is a multiple of 4
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* will be a leap year. This leads to a year that is 365.25 days long,
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* but the current accepted value for the tropical year is 365.242199
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* days.
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*
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* To correct this error in the length of the year and to bring the
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* vernal equinox back to March 21, Pope Gregory XIII issued a papal
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* bull declaring that Thursday October 4, 1582 would be followed by
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* Friday October 15, 1582 and that centennial years would only be a
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* leap year if they were a multiple of 400. This shortened the year
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* by 3 days per 400 years, giving a year of 365.2425 days.
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*
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* Another recently proposed change in the leap year rule is to make
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* years that are multiples of 4000 not a leap year, but this has never
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* been officially accepted and this rule is not implemented in these
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* algorithms.
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*
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* ALGORITHMS
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*
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* The calculations are based on three different cycles: a 400 year
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* cycle of leap years, a 4 year cycle of leap years and a 5 month
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* 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
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* and 31 days, except for three anomalies: both July and August
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* have 31 days, both December and January have 31, and February
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* is less than 30. Starting with March, the lengths are in a
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* cycle of 5 months (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
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* year 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 4714 B.C. to 10000
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* A.D. The source code of the verification program is included in
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* this 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).
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*
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**************************************************************************/
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#include "sdncal.h"
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#define GREGOR_SDN_OFFSET 32045
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#define DAYS_PER_5_MONTHS 153
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#define DAYS_PER_4_YEARS 1461
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#define DAYS_PER_400_YEARS 146097
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void SdnToGregorian(
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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 century;
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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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*pYear = 0;
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*pMonth = 0;
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*pDay = 0;
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return;
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}
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temp = (sdn + GREGOR_SDN_OFFSET) * 4 - 1;
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/* Calculate the century (year/100). */
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century = temp / DAYS_PER_400_YEARS;
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/* Calculate the year and day of year (1 <= dayOfYear <= 366). */
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temp = ((temp % DAYS_PER_400_YEARS) / 4) * 4 + 3;
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year = (century * 100) + (temp / DAYS_PER_4_YEARS);
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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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}
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long int GregorianToSdn(
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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 < -4714 ||
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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 (Nov 25, 4714 B.C.) */
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if (inputYear == -4714) {
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if (inputMonth < 11) {
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return (0);
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}
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if (inputMonth == 11 && inputDay < 25) {
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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 / 100) * DAYS_PER_400_YEARS) / 4
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+ ((year % 100) * 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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- GREGOR_SDN_OFFSET);
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}
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char *MonthNameShort[13] =
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{
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"",
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"Jan",
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"Feb",
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"Mar",
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"Apr",
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"May",
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"Jun",
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"Jul",
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"Aug",
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"Sep",
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"Oct",
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"Nov",
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"Dec"
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};
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char *MonthNameLong[13] =
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{
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"",
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"January",
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"February",
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"March",
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"April",
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"May",
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"June",
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"July",
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"August",
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"September",
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"October",
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"November",
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"December"
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};
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