Gregorian-Hijri Dates Converter JAVA

import java.util.Calendar;
/**
 * Gregorian-Hijri Dates Converter
 * 
 * 
 * This Code is used to convert Gregorian dates to Hijri Dates 
 * 
 *
 */

public class DateHigri {
	
	
	static double gmod(double n,double  m) {
		return ((n % m) + m) % m;
	}

static double[] kuwaiticalendar(boolean adjust) {
		Calendar today = Calendar.getInstance();
		int adj=0;
		if(adjust){
			adj=0;
		}else{
			adj=1;
		}
		
		if (adjust) {
			int adjustmili = 1000 * 60 * 60 * 24 * adj;
			long todaymili = today.getTimeInMillis() + adjustmili;
			today.setTimeInMillis(todaymili);
		}
		double day = today.get(Calendar.DAY_OF_MONTH);
		double  month = today.get(Calendar.MONTH);
		double  year = today.get(Calendar.YEAR);
		
		double  m = month + 1;
		double  y = year;
		if (m < 3) {
			y -= 1;
			m += 12;
		}

double a = Math.floor(y / 100.);
		double b = 2 - a + Math.floor(a / 4.);
	
		if (y < 1583)
			b = 0;
		if (y == 1582) {
			if (m > 10)
				b = -10;
			if (m == 10) {
				b = 0;
				if (day > 4)
					b = -10;
			}
		}

double jd = Math.floor(365.25 * (y + 4716)) + Math.floor(30.6001 * (m + 1)) + day
				+ b - 1524;
		
		b = 0;
		if (jd > 2299160) {
			a = Math.floor((jd - 1867216.25) / 36524.25);
			b = 1 + a - Math.floor(a / 4.);
		}
		double bb = jd + b + 1524;
		double cc = Math.floor((bb - 122.1) / 365.25);
		double dd = Math.floor(365.25 * cc);
		double ee = Math.floor((bb - dd) / 30.6001);
		day = (bb - dd) - Math.floor(30.6001 * ee);
		month = ee - 1;
		if (ee > 13) {
			cc += 1;
			month = ee - 13;
		}
		year = cc - 4716;

double wd = gmod(jd + 1, 7) + 1;

double iyear = 10631. / 30.;
		double epochastro = 1948084;
		double epochcivil = 1948085;

double shift1 = 8.01 / 60.;

double z = jd - epochastro;
		double cyc = Math.floor(z / 10631.);
		z = z - 10631 * cyc;
		double j = Math.floor((z - shift1) / iyear);
		double iy = 30 * cyc + j;
		z = z - Math.floor(j * iyear + shift1);
		double im = Math.floor((z + 28.5001) / 29.5);
		if (im == 13)
			im = 12;
		double id = z - Math.floor(29.5001 * im - 29);

double[]  myRes = new double[8];

myRes[0] = day; // calculated day (CE)
		myRes[1] = month - 1; // calculated month (CE)
		myRes[2] = year; // calculated year (CE)
		myRes[3] = jd - 1; // julian day number
		myRes[4] = wd - 1; // weekday number
		myRes[5] = id; // islamic date
		myRes[6] = im - 1; // islamic month
		myRes[7] = iy; // islamic year

return myRes;
	}
	static String writeIslamicDate() {
		String[] wdNames = {"Ahad", "Ithnin", "Thulatha", "Arbaa", "Khams",
				"Jumuah", "Sabt"};
		String[] iMonthNames = {"Muharram", "Safar", "Rabi'ul Awwal",
				"Rabi'ul Akhir", "Jumadal Ula", "Jumadal Akhira", "Rajab",
				"Sha'ban", "Ramadan", "Shawwal", "Dhul Qa'ada", "Dhul Hijja"};
		// This Value is used to give the correct day +- 1 day
		boolean dayTest=true;
		double[] iDate = kuwaiticalendar(dayTest);
		String outputIslamicDate = wdNames[(int) iDate[4]] + ", " + iDate[5] + " "
				+ iMonthNames[(int) iDate[6]] + " " + iDate[7] + " AH";
		
		return outputIslamicDate;
	}
}

Did you have a question or are you just sharing your code ?

Am just sharing the code

Ok, thanks then.

I do have some comments about the code. It contains a lot of "magic" numbers: 365.25, 4716, 30.6001, 1524, 2299160, 1867216.25, 36524.25, 122.1, 10631, 1948084, 1948085, 8.01, 28.5001, 29.5001, ...

What do all those numbers mean? How do you know this code works correctly? For which range of dates does it work correctly?

These numbers are used to calculate the Islamic Higri date, you can check if the date returned on the following site if its correct
http://www.islamicity.com/PrayerTimes/defaultHijriConv.asp

365.25 in particular sounds suspicious - does this code handle leap years correctly in the long run?

Maybe you can provide a link to Wikipedia page or something like that where the topic (not necessarily the calculations) are explained.

http://en.wikipedia.org/wiki/Islamic_calendar
http://en.wikipedia.org/wiki/Kuwaiti_algorithm

Very useful .. thank you

ghaith shamayleh wrote:These numbers are used to calculate the Islamic Higri date, you can check if the date returned on the following site if its correct
http://www.islamicity.com/PrayerTimes/defaultHijriConv.asp

Then make them constants - private static final fields of the right type (int / double) with a descriptive name.

The duration of a year is not 365.25 days. According to this webpage that is 11min 14 seconds too long. anyway, using floating-point arithmetic is bound to cause imprecisions.

Campbell Ritchie wrote:The duration of a year is not 365.25 days. According to this webpage that is 11min 14 seconds too long. anyway, using floating-point arithmetic is bound to cause imprecisions.

It's much more complicated than that.

According to Wikipedia, a Julian year is 365.25 days. I guess that the definition on the above website is the length of a Gregorian year (a year according to the usual Gregorial calendar). But there are also other ways to define a year: for example a sidereal year, which is the time it takes the Earth to make exactly one orbit around the Sun, is slightly different (365 days, 6 hours 9 minutes, 9.7676 seconds). And there are other definitions which are again slightly different.