/* $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 GREGOR_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 + GREGOR_SDN_OFFSET) * 4 - 1; if (temp < 0) { *pYear = 0; *pMonth = 0; *pDay = 0; return; } /* 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 - GREGOR_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" };