@pacote/computus
Version:
Determine the calendar date of the Easter holiday.
68 lines • 2.11 kB
JavaScript
function integerDivision(dividend, divider) {
return Math.floor(dividend / divider);
}
/**
* This function takes a year and returns a `Date` object with the Gregorian
* calendar Easter day on that year at midnight.
*
* The function uses a version of the Meeus/Jones/Butcher algorithm published
* by _New Scientist_ on 30 March 1961.
*
* @example
* ```typescript
* import { gregorian } from '@pacote/computus'
*
* gregorian(2020) // .toLocaleDateString() => '4/12/2020'
* ```
*
* @param year Year.
*
* @returns Easter date for the provided year.
*/
export function gregorian(year) {
const a = year % 19;
const b = integerDivision(year, 100);
const c = year % 100;
const d = integerDivision(b, 4);
const e = b % 4;
const g = integerDivision(8 * b + c, 25);
const h = (19 * a + b - d - g + 15) % 30;
const i = integerDivision(c, 4);
const k = c % 4;
const l = (2 * e + 2 * i - h - k + 32) % 7;
const m = integerDivision(a + 11 * h + 19 * l, 433);
const n = integerDivision(h + l - 7 * m + 90, 25);
const p = (h + l - 7 * m + 33 * n + 19) % 32;
return new Date(year, n - 1, p);
}
/**
* This function takes a year and returns a `Date` object with the Eastern
* Orthodox Easter day on that year at midnight. Please note that this date
* is returned for the Gregorian calendar, 13 days (as of 1900 through 2099)
* after the Julian date.
*
* The function implements the Jean Meeus algorithm from his book
* _Astronomical Algorithms_ (1991).
*
* @example
* ```typescript
* import { julian } from '@pacote/computus'
*
* julian(2020) // .toLocaleDateString() => '4/19/2020'
* ```
*
* @param year Year.
*
* @returns Eastern Orthodox Easter date for the provided year.
*/
export function julian(year) {
const a = year % 4;
const b = year % 7;
const c = year % 19;
const d = (19 * c + 15) % 30;
const e = (2 * a + 4 * b - d + 34) % 7;
const month = integerDivision(d + e + 114, 31);
const day = ((d + e + 114) % 31) + 1;
return new Date(year, month - 1, day + 13);
}
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