// Copyright 2018-2025 the Deno authors. MIT license.
// This module is browser compatible.
function isYearNumberALeapYear(yearNumber: number): boolean {
return (
(yearNumber % 4 === 0 && yearNumber % 100 !== 0) || yearNumber % 400 === 0
);
}
/**
* Returns whether the given year is a leap year. Passing in a
* {@linkcode Date} object will return the leap year status of the year of that
* object and take the current timezone into account. Passing in a number will
* return the leap year status of that number.
*
* This is based on
* {@link https://docs.microsoft.com/en-us/office/troubleshoot/excel/determine-a-leap-year}.
*
* @param year The year in number or `Date` format.
* @returns `true` if the given year is a leap year; `false` otherwise.
*
* @example Basic usage
* ```ts
* import { isLeap } from "@std/datetime/is-leap";
* import { assertEquals } from "@std/assert";
*
* assertEquals(isLeap(new Date("1970-01-02")), false);
*
* assertEquals(isLeap(1970), false);
*
* assertEquals(isLeap(new Date("1972-01-02")), true);
*
* assertEquals(isLeap(1972), true);
* ```
*
* @example Accounting for timezones
* ```ts no-assert
* import { isLeap } from "@std/datetime/is-leap";
*
* // True if the local timezone is GMT+0; false if the local timezone is GMT-1
* isLeap(new Date("2000-01-01"));
*
* // True regardless of the local timezone
* isLeap(2000);
*
* ```
*/
export function isLeap(year: Date | number): boolean {
const yearNumber = year instanceof Date ? year.getFullYear() : year;
return isYearNumberALeapYear(yearNumber);
}
/**
* Returns whether the given year is a leap year in UTC time. This always
* returns the same value regardless of the local timezone.
* This is based on
* {@link https://docs.microsoft.com/en-us/office/troubleshoot/excel/determine-a-leap-year}.
*
* @param year The year in number or `Date` format.
* @returns `true` if the given year is a leap year; `false` otherwise.
*
* @example Basic usage
* ```ts
* import { isUtcLeap } from "@std/datetime/is-leap";
* import { assertEquals } from "@std/assert";
*
* assertEquals(isUtcLeap(new Date("2000-01-01")), true);
*
* assertEquals(isUtcLeap(new Date("December 31, 1999 23:59:59 GMT-01:00")), true);
*
* assertEquals(isUtcLeap(2000), true);
*
* assertEquals(isUtcLeap(1999), false);
* ```
*/
export function isUtcLeap(year: Date | number): boolean {
const yearNumber = year instanceof Date ? year.getUTCFullYear() : year;
return isYearNumberALeapYear(yearNumber);
}
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