Skip to content

Instantly share code, notes, and snippets.

@johndyer
Last active Apr 21, 2022
Embed
What would you like to do?
Calculate Easter in JavaScript
/**
* 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];
}
@DHFW
Copy link

DHFW commented May 29, 2019

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).

@varvino
Copy link

varvino commented Feb 6, 2020

Thanks a lot for this

@gavinr
Copy link

gavinr commented Mar 10, 2020

I am using this here: Easter Dates
Thanks!

@johndyer
Copy link
Author

johndyer commented Mar 10, 2020

@sunsero
Copy link

sunsero commented Apr 11, 2020

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

@johndyer
Copy link
Author

johndyer commented Apr 11, 2020

@sunsero
Copy link

sunsero commented Apr 11, 2020

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.

@johndyer
Copy link
Author

johndyer commented Apr 11, 2020

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

@sunsero
Copy link

sunsero commented Apr 11, 2020

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.

@Termplexed
Copy link

Termplexed commented Mar 1, 2021

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,

@johndyer
Copy link
Author

johndyer commented Mar 1, 2021

Thanks @Termplexed! Fixed.

@creat73
Copy link

creat73 commented Apr 1, 2021

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.

@wysal
Copy link

wysal commented Jun 9, 2021

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

@tdiluzio
Copy link

tdiluzio commented Nov 22, 2021

Thanks @johndyer and @wysal 👍

@P06316
Copy link

P06316 commented Nov 26, 2021

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

@PenkoMlakar
Copy link

PenkoMlakar commented Mar 18, 2022

Thanks @johndyer!

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment