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];
}

Thank you!

For others: be aware that this returns the month as 1 = January, 2 = February and so on.

I subtracted the month with one to be able to convert it to a Javascript Date easily in which the month is index based (0 = January, 1 = February).

Thanks a lot for this

I am using this here: Easter Dates
Thanks!

Thanks for sharing! That is incredible! JD

…

On Tue, Mar 10, 2020 at 9:51 AM Gavin Rehkemper ***@***.***> wrote: I am using this here: Easter Dates <https://easter-dates.gavinr.com/> Thanks! — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <https://gist.github.com/0dffbdd98c2046f41180c051f378f343?email_source=notifications&email_token=AABUJUZCKAW35GHFPZSSI7LRGZHX5A5CNFSM4HQMLXF2YY3PNVWWK3TUL52HS4DFVNDWS43UINXW23LFNZ2KUY3PNVWWK3TUL5UWJTQAGDXLW#gistcomment-3206843>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AABUJU6SR4EAJIYK3EJC4RLRGZHX5ANCNFSM4HQMLXFQ> .

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

Is it just me or is the function returning Easter Monday this year? Happy Easter, anyway!

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