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From: rjh@ihlpa.ATT.COM (45261-Herber)
Newsgroups: comp.misc
Subject: Re: Julian Date/Day Number
Message-ID: <6205@ihlpa.ATT.COM>
Date: Tue, 10-Nov-87 10:31:19 EST
Article-I.D.: ihlpa.6205
Posted: Tue Nov 10 10:31:19 1987
Date-Received: Fri, 13-Nov-87 05:35:34 EST
References: <557@astroatc.UUCP>
Reply-To: rjh@ihlpa.UUCP (45261-Herber,R.J.)
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 113
Keywords: Ceasar, Astronomy, Dates, Etc.

Astronomical Julian Dates/Day Number number from January 1, 4713 BC.
That date was picked because a number of various different calendar
cycles come to their initial points then and for most purposes is
sufficiently far into the past that all {:-)} useful dates are
positive numbers.
I am including a program that computes the astronomical Julian Date
using the Roman Catholic church's definition of when the switch
between the Julian and the Georgian calendars occurred, i.e. Oct 4,
1582 Old Style was followed by Oct 15, 1582 New Style.

	Randolph J. Herber, Amdahl Sr Sys Eng, ..!ihnp4!ihlpa!rjh,
	(312) 979-6553, IH 6X213, AT&T Bell Labs, Naperville, IL 60566

#include <stdio.h>
#include <time.h>
/*
This program writes a astronomical Julian date (to stdout). 
If no argument is specified, it calls a "ctime"
function to use today's date.  Otherwise, it may be invoked
with 3 arguments -- month, day, year (4-digit).
*/

#define TRUE     1
#define FALSE    0

static int mlen[] = {
	0,31,28,31,30,31,30,31,31,30,31,30,31
};

int unit(f1,f2)
	int f1, f2;
{
	int rmdr;
	rmdr = f1 % f2;
	if (rmdr < 0) rmdr += f2;
	return (f1 - rmdr) / f2;
}

main(argc,argv)
int argc;
char **argv;
{

	long secs, time();
	struct tm *tbuf, *localtime();
	void exit();
	int year, yr, mo, da, unit();
	long jdate;
	short yrm1, n;
	char style,leap;

	if (argc > 1 && argc != 4) {
		(void) printf("Usage:  No arguments for today's date,");
		(void) printf(" or '%s <mo> <da> <yr>'.\n", argv[0]);
		(void) exit(1);
	}
	else if (argc == 1) {
		secs = time((long *)0);
		tbuf = localtime(&secs);
		mo = tbuf->tm_mon + 1;
		da = tbuf->tm_mday;
		year = tbuf->tm_year + 1900;
	}
	else {
		mo = atoi(argv[1]);
		da = atoi(argv[2]);
		year = atoi(argv[3]);
	}

	if (year != 0 && year > -10000 && year < 10000) {
		yr = year > 0 ? year : year + 1;
		style = year > 1582;
		leap = style
		    ? (((yr % 4) == 0) && ((yr % 100) != 0)) ||
			((yr % 400) == 0)
		    : ((yr % 4) == 0);
		mlen[2] = leap ? 29 : 28;
		if (mo >=1 && mo <= 12 && da >= 1 && da <= mlen[mo] &&
		    !(year == 1582 && (mo == 1 || mo == 2 ||
		    (mo == 10 && da >= 5 && da <= 14)))) {
			if (year == 1582 && mo == 10) {
				mlen[10] = 21;
				if (da >= 15) da -= 10;
			} else {
				mlen[10] = 31;
			}
			yrm1 = yr - 1;
			jdate = 1721423l+yrm1*365+unit(yrm1,4)+da;
			for (n=1;n<mo;++n) {
				jdate += mlen[n];
			}
			if (style) {
				jdate -= 10 + (yrm1-1500)/100 - (yrm1-1200)/400;
			}
			(void) printf("%ld\n",jdate);
			(void) exit(0);
		} else {
			(void) printf("%s: Invalid date!\n",*argv);
			(void) exit(1);
		}
	}
	(void) printf("%s: Date out of range!\n",*argv);
	(void) exit(2);
}

int unit(f1,f2)
	int f1, f2;
{
	int rmdr;
	rmdr = f1 % f2;
	if (rmdr < 0) rmdr += f2;
	return (f1 - rmdr) / f2;
}