/* $selId: gregor.c,v 2.0 1995/10/24 01:13:06 lees Exp $
* Copyright 1993-1995, Scott E. Lee, all rights reserved.
* Permission granted to use, copy, modify, distribute and sell so long as
* the above copyright and this permission statement are retained in all
* copies. THERE IS NO WARRANTY - USE AT YOUR OWN RISK.
*/
/**************************************************************************
*
* These are the externally visible components of this file:
*
* void
* 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
* 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 *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 *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.
*
* 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).
*
**************************************************************************/
#include "sdncal.h"
#define SDN_OFFSET 32045
#define DAYS_PER_5_MONTHS 153
#define DAYS_PER_4_YEARS 1461
#define DAYS_PER_400_YEARS 146097
void
SdnToGregorian(
long int sdn,
int *pYear,
int *pMonth,
int *pDay)
{
int century;
int year;
int month;
int day;
long int temp;
int dayOfYear;
if (sdn <= 0) {
*pYear = 0;
*pMonth = 0;
*pDay = 0;
return;
}
temp = (sdn + SDN_OFFSET) * 4 - 1;
/* Calculate the century (year/100). */
century = temp / DAYS_PER_400_YEARS;
/* Calculate the year and day of year (1 <= dayOfYear <= 366). */
temp = ((temp % DAYS_PER_400_YEARS) / 4) * 4 + 3;
year = (century * 100) + (temp / DAYS_PER_4_YEARS);
dayOfYear = (temp % DAYS_PER_4_YEARS) / 4 + 1;
/* Calculate the month and day of month. */
temp = dayOfYear * 5 - 3;
month = temp / DAYS_PER_5_MONTHS;
day = (temp % DAYS_PER_5_MONTHS) / 5 + 1;
/* Convert to the normal beginning of the year. */
if (month < 10) {
month += 3;
} else {
year += 1;
month -= 9;
}
/* Adjust to the B.C./A.D. type numbering. */
year -= 4800;
if (year <= 0) year--;
*pYear = year;
*pMonth = month;
*pDay = day;
}
long int
GregorianToSdn(
int inputYear,
int inputMonth,
int inputDay)
{
int year;
int month;
/* check for invalid dates */
if (inputYear == 0 || inputYear < -4714 ||
inputMonth <= 0 || inputMonth > 12 ||
inputDay <= 0 || inputDay > 31)
{
return(0);
}
/* check for dates before SDN 1 (Nov 25, 4714 B.C.) */
if (inputYear == -4714) {
if (inputMonth < 11) {
return(0);
}
if (inputMonth == 11 && inputDay < 25) {
return(0);
}
}
/* Make year always a positive number. */
if (inputYear < 0) {
year = inputYear + 4801;
} else {
year = inputYear + 4800;
}
/* Adjust the start of the year. */
if (inputMonth > 2) {
month = inputMonth - 3;
} else {
month = inputMonth + 9;
year--;
}
return( ((year / 100) * DAYS_PER_400_YEARS) / 4
+ ((year % 100) * DAYS_PER_4_YEARS) / 4
+ (month * DAYS_PER_5_MONTHS + 2) / 5
+ inputDay
- SDN_OFFSET );
}
char *MonthNameShort[13] = {
"",
"Jan",
"Feb",
"Mar",
"Apr",
"May",
"Jun",
"Jul",
"Aug",
"Sep",
"Oct",
"Nov",
"Dec"
};
char *MonthNameLong[13] = {
"",
"January",
"February",
"March",
"April",
"May",
"June",
"July",
"August",
"September",
"October",
"November",
"December"
};
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