calendar.js

/*
 JavaScript functions for the Fourmilab Calendar Converter

 by John Walker  --  September, MIM
 http://www.fourmilab.ch/documents/calendar/

 This program is in the public domain.
 */

/*  MOD  --  Modulus function which works for non-integers.  */

function mod(a, b) {
	return a - (b * Math.floor(a / b));
}

//  LEAP_GREGORIAN  --  Is a given year in the Gregorian calendar a leap year ?

function leap_gregorian(year) {
	return ((year % 4) == 0) &&
		(!(((year % 100) == 0) && ((year % 400) != 0)));
}

//  GREGORIAN_TO_JD  --  Determine Julian day number from Gregorian calendar date

var GREGORIAN_EPOCH = 1721425.5;

function gregorian_to_jd(year, month, day) {
	return (GREGORIAN_EPOCH - 1) +
		(365 * (year - 1)) +
		Math.floor((year - 1) / 4) +
		(-Math.floor((year - 1) / 100)) +
		Math.floor((year - 1) / 400) +
		Math.floor((((367 * month) - 362) / 12) +
			((month <= 2) ? 0 :
				(leap_gregorian(year) ? -1 : -2)
				) +
			day);
}

//  JD_TO_GREGORIAN  --  Calculate Gregorian calendar date from Julian day

function jd_to_gregorian(jd) {
	var wjd, depoch, quadricent, dqc, cent, dcent, quad, dquad,
		yindex, year, yearday, leapadj;

	wjd = Math.floor(jd - 0.5) + 0.5;
	depoch = wjd - GREGORIAN_EPOCH;
	quadricent = Math.floor(depoch / 146097);
	dqc = mod(depoch, 146097);
	cent = Math.floor(dqc / 36524);
	dcent = mod(dqc, 36524);
	quad = Math.floor(dcent / 1461);
	dquad = mod(dcent, 1461);
	yindex = Math.floor(dquad / 365);
	year = (quadricent * 400) + (cent * 100) + (quad * 4) + yindex;
	if (!((cent == 4) || (yindex == 4))) {
		year++;
	}
	yearday = wjd - gregorian_to_jd(year, 1, 1);
	leapadj = ((wjd < gregorian_to_jd(year, 3, 1)) ? 0
		:
		(leap_gregorian(year) ? 1 : 2)
		);
	var month = Math.floor((((yearday + leapadj) * 12) + 373) / 367),
		day = (wjd - gregorian_to_jd(year, month, 1)) + 1;

	return [year, month, day];
}


var PERSIAN_EPOCH = 1948320.5;

//  PERSIAN_TO_JD  --  Determine Julian day from Persian date

function persian_to_jd(year, month, day) {
	var epbase, epyear;

	epbase = year - ((year >= 0) ? 474 : 473);
	epyear = 474 + mod(epbase, 2820);

	return day +
		((month <= 7) ?
			((month - 1) * 31) :
			(((month - 1) * 30) + 6)
			) +
		Math.floor(((epyear * 682) - 110) / 2816) +
		(epyear - 1) * 365 +
		Math.floor(epbase / 2820) * 1029983 +
		(PERSIAN_EPOCH - 1);
}

//  JD_TO_PERSIAN  --  Calculate Persian date from Julian day

function jd_to_persian(jd) {
	var year, month, day, depoch, cycle, cyear, ycycle,
		aux1, aux2, yday;


	jd = Math.floor(jd) + 0.5;

	depoch = jd - persian_to_jd(475, 1, 1);
	cycle = Math.floor(depoch / 1029983);
	cyear = mod(depoch, 1029983);
	if (cyear == 1029982) {
		ycycle = 2820;
	} else {
		aux1 = Math.floor(cyear / 366);
		aux2 = mod(cyear, 366);
		ycycle = Math.floor(((2134 * aux1) + (2816 * aux2) + 2815) / 1028522) +
			aux1 + 1;
	}
	year = ycycle + (2820 * cycle) + 474;
	if (year <= 0) {
		year--;
	}
	yday = (jd - persian_to_jd(year, 1, 1)) + 1;
	month = (yday <= 186) ? Math.ceil(yday / 31) : Math.ceil((yday - 6) / 30);
	day = (jd - persian_to_jd(year, month, 1)) + 1;
	return [year, month, day];
}