--- /dev/null
+/* This file is taken from http://www.genealogy.org/~scottlee/
+ Only this initial comment has been added. The next comment
+ gives the original copyright notice.
+*/
+
+
+/* $selId: julian.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
+ * 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
+ * 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.
+ *
+ * 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 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). [Note: the published algorithm is for the
+ * Gregorian calendar, but was adjusted to use the Julian calendar's
+ * simpler leap year rule.]
+ *
+ **************************************************************************/
+
+#include "sdncal.h"
+
+#define SDN_OFFSET 32083
+#define DAYS_PER_5_MONTHS 153
+#define DAYS_PER_4_YEARS 1461
+
+void
+SdnToJulian(
+ long int sdn,
+ int *pYear,
+ int *pMonth,
+ int *pDay)
+{
+ 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 year and day of year (1 <= dayOfYear <= 366). */
+ year = 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
+JulianToSdn(
+ int inputYear,
+ int inputMonth,
+ int inputDay)
+{
+ int year;
+ int month;
+
+ /* check for invalid dates */
+ if (inputYear == 0 || inputYear < -4713 ||
+ inputMonth <= 0 || inputMonth > 12 ||
+ inputDay <= 0 || inputDay > 31)
+ {
+ return(0);
+ }
+
+ /* check for dates before SDN 1 (Jan 2, 4713 B.C.) */
+ if (inputYear == -4713) {
+ if (inputMonth == 1 && inputDay == 1) {
+ 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 * DAYS_PER_4_YEARS) / 4
+ + (month * DAYS_PER_5_MONTHS + 2) / 5
+ + inputDay
+ - SDN_OFFSET );
+}
projects / gedcom-parse.git / blobdiff