Calculate Easter in JavaScript

easter.js

/**
* Calculates Easter in the Gregorian/Western (Catholic and Protestant) calendar 
* based on the algorithm by Oudin (1940) from http://www.tondering.dk/claus/cal/easter.php
* @returns {array} [int month, int day]
*/
function getEaster(year) {
	var f = Math.floor,
		// Golden Number - 1
		G = year % 19,
		C = f(year / 100),
		// related to Epact
		H = (C - f(C / 4) - f((8 * C + 13)/25) + 19 * G + 15) % 30,
		// number of days from 21 March to the Paschal full moon
		I = H - f(H/28) * (1 - f(29/(H + 1)) * f((21-G)/11)),
		// weekday for the Paschal full moon
		J = (year + f(year / 4) + I + 2 - C + f(C / 4)) % 7,
		// number of days from 21 March to the Sunday on or before the Paschal full moon
		L = I - J,
		month = 3 + f((L + 40)/44),
		day = L + 28 - 31 * f(month / 4);

	return [month,day];
}

Timezone issue ?

On Sat, Apr 11, 2020 at 11:46 AM sunsero ***@***.***> wrote: ***@***.**** commented on this gist. ------------------------------ Is it just me or is the function returning Easter Monday this year? Happy Easter, anyway! — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <https://gist.github.com/0dffbdd98c2046f41180c051f378f343#gistcomment-3250261>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AABUJU6GQ6CF4TI3TQYBBG3RMCNIHANCNFSM4HQMLXFQ> .

-- John Dyer - http://j.hn/

Don't think so because Easter Sunday is calculated for 2019 and for 2021 correctly. If it matters I'm during summer time in UTC+2, in winter in UTC+1.

getEaster(2020);
returns
[4, 12]
for me...

Thanks, you're right - if I pass 2020 as a constant value rather than in a variable it gives the correct result. I'll have to figure out what's going on.

Minor nuisance. On line 12 there is thin-space in stead of normal space. (0x2009 instead of 0x20) in C - f

		H = (C<0x2009>-<0x2009>f(C / 4) - f((8 * C + 13)/25) + 19 * G + 15) % 30,

Thanks @Termplexed! Fixed.

This algorithm seems to fail for year 2022 returning 23.04 instead of 17.04. Works fine for any other year I tried. Any idea on why that might be? I'm really curious and couldn't find explanation.

I needed to count days up from easter for public holidays calculation so I did some adaptions:

function getEaster(year, plusdays) {
var f = Math.floor,
G = year % 19,
C = f(year / 100),
H = (C - f(C / 4) - f((8 * C + 13) / 25) + 19 * G + 15) % 30,
I = H - f(H / 28) * (1 - f(29 / (H + 1)) * f((21 - G) / 11)),
J = (year + f(year / 4) + I + 2 - C + f(C / 4)) % 7,
L = I - J,
month = 3 + f((L + 40) / 44),
day = L + 28 - 31 * f(month / 4);
return new Date(year, month - 1, day + plusdays);
}

so this would then output for example Easter Monday:
getEaster(2022, 1)

in case of need for someone

Thanks @johndyer and @wysal 👍

This algorithm seems to fail for year 2022 returning 23.04 instead of 17.04. Works fine for any other year I tried. Any idea on why that might be? I'm really curious and couldn't find explanation.

I had the same issue and solved it by applying parseInt(myYear, 0) on the variable containing the year.

console.log(getEaster("2022")); //Returns [4, 23] => NOK
console.log(getEaster(2022)); //Returns [4, 17] => OK
console.log(getEaster(parseInt("2022", 0))); //Returns [4, 17] => OK

Thanks @johndyer!

Thanks seem to work!

Was wondering if it's possible to get the good Friday from this? Since it's the Friday before Easter.

So I guess day - 2 will do for most cases at least.

When using this, I was passing the year in from an form input and it seems as though, only odd numbered years, returned the correct date (being Easter Sunday). The answer was to ensure the value passed in was an integer so added a line at the top of the function.

year = parseInt(year)