How can I calculate the date in JavaScript knowing the week number and the year? For week number 20 and year 2013 I want to obtain 2013-05-16. I am trying it like this:
Date.prototype.dayofYear = function () {
var d = new Date(this.getFullYear(), 0, 0)
return Math.floor((/* enter code here */ this - d) / 8.64e + 7)
}
function getDateOfWeek(w, y) {
var d = (1 + (w - 1) * 7); // 1st of January + 7 days for each week
return new Date(y, 0, d);
}
This uses the simple week definition, meaning the 20th week of 2013 is May 14.
To calculate the date of the start of a given ISO8601 week (which will always be a Monday) either use Moment.js, e.g.:
moment("2013W20").toDate()
or without external libraries:
/**
* Get the date from an ISO 8601 week and year
*
* https://en.wikipedia.org/wiki/ISO_week_date
*
* @param {number} week ISO 8601 week number
* @param {number} year ISO year
*
* Examples:
* getDateOfIsoWeek(53, 1976) -> Mon Dec 27 1976
* getDateOfIsoWeek( 1, 1978) -> Mon Jan 02 1978
* getDateOfIsoWeek( 1, 1980) -> Mon Dec 31 1979
* getDateOfIsoWeek(53, 2020) -> Mon Dec 28 2020
* getDateOfIsoWeek( 1, 2021) -> Mon Jan 04 2021
* getDateOfIsoWeek( 0, 2023) -> Invalid (no week 0)
* getDateOfIsoWeek(53, 2023) -> Invalid (no week 53 in 2023)
*/
function getDateOfIsoWeek(week, year) {
week = parseFloat(week);
year = parseFloat(year);
if (week < 1 || week > 53) {
throw new RangeError("ISO 8601 weeks are numbered from 1 to 53");
} else if (!Number.isInteger(week)) {
throw new TypeError("Week must be an integer");
} else if (!Number.isInteger(year)) {
throw new TypeError("Year must be an integer");
}
const simple = new Date(year, 0, 1 + (week - 1) * 7);
const dayOfWeek = simple.getDay();
const isoWeekStart = simple;
// Get the Monday past, and add a week if the day was
// Friday, Saturday or Sunday.
isoWeekStart.setDate(simple.getDate() - dayOfWeek + 1);
if (dayOfWeek > 4) {
isoWeekStart.setDate(isoWeekStart.getDate() + 7);
}
// The latest possible ISO week starts on December 28 of the current year.
if (isoWeekStart.getFullYear() > year ||
(isoWeekStart.getFullYear() == year &&
isoWeekStart.getMonth() == 11 &&
isoWeekStart.getDate() > 28)) {
throw new RangeError(`${year} has no ISO week ${week}`);
}
return isoWeekStart;
}
Result: the 20th week of 2013 is May 13, which can be confirmed here.
Sorry if this is a little verbose but this solution will also return the Sunday after calculating the week number. It combines answers I've seen from a couple different places:
function getSundayFromWeekNum(weekNum, year) {
var sunday = new Date(year, 0, (1 + (weekNum - 1) * 7));
while (sunday.getDay() !== 0) {
sunday.setDate(sunday.getDate() - 1);
}
return sunday;
}
2019 version (if anyone wants to calculate the date of week's start)
The answer of Elle is pretty much right, but there is one exception: if you want to get a date of start of the week - it won't help you because Elle's algorithm doesn't mind the day, from which the week starts of, at all.
And here is why:
Elle's solution
function getDateOfWeek(w, y) {
var d = (1 + (w - 1) * 7); // 1st of January + 7 days for each week
return new Date(y, 0, d);
}
Here you basically just calculating the amount of days from the start of the year (1st January). This means that it doesn't matter from which day your year starts of: Monday, Sunday or Friday - it will not make any difference.
To set the day, from which your week will start of (let's say Monday, for example), you just need to:
function getDateOfWeek(w, y) {
let date = new Date(y, 0, (1 + (w - 1) * 7)); // Elle's method
date.setDate(date.getDate() + (1 - date.getDay())); // 0 - Sunday, 1 - Monday etc
return date
}
ISO Weeks
function weekDateToDate (year, week, day) {
const firstDayOfYear = new Date(year, 0, 1)
const days = 2 + day + (week - 1) * 7 - firstDayOfYear.getDay()
return new Date(year, 0, days)
}
Get date range according to week number
Date.prototype.getWeek = function () {
var target = new Date(this.valueOf());
var dayNr = (this.getDay() + 6) % 7;
target.setDate(target.getDate() - dayNr + 3);
var firstThursday = target.valueOf();
target.setMonth(0, 1);
if (target.getDay() != 4) {
target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
}
return 1 + Math.ceil((firstThursday - target) / 604800000);
}
function getDateRangeOfWeek(weekNo){
var d1 = new Date();
numOfdaysPastSinceLastMonday = eval(d1.getDay()- 1);
d1.setDate(d1.getDate() - numOfdaysPastSinceLastMonday);
var weekNoToday = d1.getWeek();
var weeksInTheFuture = eval( weekNo - weekNoToday );
d1.setDate(d1.getDate() + eval( 7 * weeksInTheFuture ));
var rangeIsFrom = eval(d1.getMonth()+1) +"/" + d1.getDate() + "/" + d1.getFullYear();
d1.setDate(d1.getDate() + 6);
var rangeIsTo = eval(d1.getMonth()+1) +"/" + d1.getDate() + "/" + d1.getFullYear() ;
return rangeIsFrom + " to "+rangeIsTo;
};
getDateRangeOfWeek(10)
"3/6/2017 to 3/12/2017"
getDateRangeOfWeek(30)
"7/24/2017 to 7/30/2017"
function getDateOfWeek(weekNumber,year){
//Create a date object starting january first of chosen year, plus the number of days in a week multiplied by the week number to get the right date.
return new Date(year, 0, 1+((weekNumber-1)*7));
}
var myDate = getDateOfWeek(20,2013);
ISO week date (e.g. 2017-W32-3) to normal date:
const getZeroBasedIsoWeekDay = date => (date.getDay() + 6) % 7
const getIsoWeekDay = date => getZeroBasedIsoWeekDay(date) + 1
function weekDateToDate(year, week, weekDay) {
const zeroBasedWeek = week - 1
const zeroBasedWeekDay = weekDay - 1
let days = (zeroBasedWeek * 7) + zeroBasedWeekDay
// Dates start at 2017-01-01 and not 2017-01-00
days += 1
const firstDayOfYear = new Date(year, 0, 1)
const firstIsoWeekDay = getIsoWeekDay(firstDayOfYear)
const zeroBasedFirstIsoWeekDay = getZeroBasedIsoWeekDay(firstDayOfYear)
// If year begins with W52 or W53
if (firstIsoWeekDay > 4) days += 8 - firstIsoWeekDay
// Else begins with W01
else days -= zeroBasedFirstIsoWeekDay
return new Date(year, 0, days)
}
const expectedAndActual = [
[new Date('2007-01-01'), weekDateToDate(2007, 1, 1)],
[new Date('2015-11-24'), weekDateToDate(2015, 48, 2)],
[new Date('2015-12-31'), weekDateToDate(2015, 53, 4)],
[new Date('2016-01-03'), weekDateToDate(2015, 53, 7)],
[new Date('2017-01-01'), weekDateToDate(2016, 52, 7)],
[new Date('2017-01-02'), weekDateToDate(2017, 1, 1)],
[new Date('2017-05-07'), weekDateToDate(2017, 18, 7)],
[new Date('2018-12-31'), weekDateToDate(2019, 1, 1)],
]
expectedAndActual
.forEach(value => {
const expected = value[0].toISOString()
const actual = value[1].toISOString()
const isEqual = actual === expected
console.assert(isEqual, '%s !== %s', actual, expected)
console.log(actual, '===', expected)
})@adius function worked for me but I made some improvements for people who are looking for a simple copy paste solution:
/**
* get date by week number
* @param {Number} year
* @param {Number} week
* @param {Number} day of week (optional; default = 0 (Sunday))
* @return {Date}
*/
function weekToDate(year, week, weekDay = 0) {
const getZeroBasedIsoWeekDay = date => (date.getDay() + 6) % 7;
const getIsoWeekDay = date => getZeroBasedIsoWeekDay(date) + 1;
const zeroBasedWeek = week - 1;
const zeroBasedWeekDay = weekDay - 1;
let days = (zeroBasedWeek * 7) + zeroBasedWeekDay;
// Dates start at 2017-01-01 and not 2017-01-00
days += 1;
const firstDayOfYear = new Date(year, 0, 1);
const firstIsoWeekDay = getIsoWeekDay(firstDayOfYear);
const zeroBasedFirstIsoWeekDay = getZeroBasedIsoWeekDay(firstDayOfYear);
// If year begins with W52 or W53
if (firstIsoWeekDay > 4) {
days += 8 - firstIsoWeekDay;
// Else begins with W01
} else {
days -= zeroBasedFirstIsoWeekDay;
}
return new Date(year, 0, days);
}
// test:
const x = [
{year: 2001, week: 10},
{year: 2019, week: 7},
{year: 2020, week: 5},
{year: 2020, week: 1},
{year: 2020, week: 12},
{year: 2020, week: 2},
{year: 2020, week: 40},
{year: 2021, week: 4},
{year: 2021, week: 3},
{year: 2021, week: 2},
{year: 2032, week: 1}
]
for (let i = 0; i < x.length; i++) {
let date = weekToDate(x[i].year, x[i].week, 0);
let d = new Date(date).toLocaleDateString('en-US', {
year: 'numeric',
month: 'long',
day: 'numeric'
})
console.log(d);
}
This is for getting the first monday of a week in the current year (ISO 8601):
function getFirstMondayOfWeek(weekNo) {
var firstMonday = new Date(new Date().getFullYear(), 0, 4, 0, 0, 0, 0);
while (firstMonday.getDay() != 1) {
firstMonday.setDate(firstMonday.getDate() - 1);
}
if (1 <= weekNo && weekNo <= 52)
return firstMonday.setDate(firstMonday.getDate() + 7 * (weekNo - 1));
firstMonday.setDate(firstMonday.getDate() + 7 * (weekNo - 1));
if (weekNo = 53 && firstMonday.getDate() >= 22 && firstMonday.getDate() <= 28)
return firstMonday;
return null;
}
I just created this one:
//Returns monday and saturday of the week
function getWeekRange(weekNo,YearNo) {
let firstDayofYear = new Date(yearNo, 0, 1);
if (firstDayofYear.getDay() > 4) {
let weekStart = new Date(yearNo, 0, 1 + (weekNo - 1) * 7 - firstDayofYear.getDay() + 8);
let weekEnd = new Date(yearNo, 0, 1 + (weekNo - 1) * 7 - firstDayofYear.getDay() + 8 + 5);
return { startDay: weekStart, endDay: weekEnd }
}
else {
let weekStart = new Date(yearNo, 0, 1 + (weekNo - 1) * 7 - firstDayofYear.getDay() + 1);
let weekEnd = new Date(yearNo, 0, 1 + (weekNo - 1) * 7 - firstDayofYear.getDay() + 1 + 5);
return { startDay: weekStart, endDay: weekEnd }
}
}
This is a short way
function addWeeks(numOfWeeks, date = new Date()) {
date.setDate(date.getDate() + numOfWeeks * 7);
return date;
}
Ex: Today is 2022-08-02
addWeeks(1)
the result will be today + 7
Tue Aug 09 2022 21:36:08 GMT+0100 (GMT+02:00)
but you may want only date without all info
addWeeks(1).toISOString().split('T')[0]
the result will be: 2022-08-09
ISO weeks (https://en.wikipedia.org/wiki/ISO_week_date):
const y = 2023;
const jan1 = new Date(y, 0, 1);
const jan1Day = jan1.getDay();
const daysToMonday = jan1Day === 1? 0 : jan1Day === 0? 1 : 8 - jan1Day
console.log('jan1', jan1)
console.log('jan1Day', jan1Day)
console.log('daysToMonday', daysToMonday)
// first Monday of the year
const firstMonday = daysToMonday === 0 ? jan1 : new Date(+jan1 + daysToMonday * 86400e3);
console.log('firstMonday', firstMonday)
// get monday of a specific week (20)
console.log('monday of week 20', new Date(+firstMonday + (20 - 1) * 7 * 86400e3))
// get sunday of a specific week (20)
console.log('sunday of week 20', new Date(+firstMonday + ((20 - 1) * 7 * 86400e3) + (86400e3 * 6) ))
The following function is adapted from https://en.wikipedia.org/wiki/ISO_week_date
It takes an ISO week date like the one you can retrieve from an input type="week" value (ie: 2023-W28) and returns you the date of that week's monday. It should be equal to valueAsDate of the same input.
It may be used as a polyfill in browser that doesn't support valueAsDate and will return an invalid date object for invalid values.
/** return date object for monday of given iso week number (\d\d\d\d-W\d\d) */
export const isoWeekDate = (isoWeek:string) => {
// alogrythm adapted from https://en.wikipedia.org/wiki/ISO_week_date
const [, Y, W] = isoWeek.match(/^(\d{4})-W(\d{2})$/) || [];
if (!Y || !W || Number(W) > 53 || Number(W) < 1) {
return new Date('x'); // Return an invalid date
}
const date = new Date(`${Y}-01-01T00:00Z`);
const jan4th = new Date(`${Y}-01-04T00:00Z`);
const jan4thDay = (jan4th.getDay() + 6) % 7;
const ordinalDate = 1 + (Number(W)-1) * 7 - jan4thDay + 3;
date.setUTCDate(ordinalDate);
if (date.getUTCFullYear() > Number(Y)) {
return new Date('x'); // Return an invalid date
}
return date
}
hope this will make someone's life easier in the future (probably me ).
Current accepted answer is close but not entirely correct. Here is the correct function:
function weekToDate(year, week) {
const days = 4 + 7 * (week - 1);
const date = new Date(year, 0, days);
return date;
}
This will yield a date that is guaranteed to be in the ISO week you asked for.
Then if you want a specific day in the week modify the date like so:
const dayOfWeek = 1 // monday
date.setDate(date.getDate() - date.getDay() + dayOfWeek)
Where dayOfWeek is the day of the week you want to get the date of. (1-7, 1 = monday)
