# javascript calculate date from week number

How i can calculate the date in javascript knowing weeknumber and the year? For week number 20 and year 2013 to obtain 5/16/2013 I am trying so:

``````Date.prototype.dayofYear = function() {
var d = new Date(this.getFullYear(), 0, 0);
return Math.floor((enter code here this - d) / 8.64e + 7);
}
``````

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)
}
``````

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;
}
``````

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"
``````

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;
}
``````

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 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)
})``````

``````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)

``````function getDateOfISOWeek(w, y) {
var simple = new Date(y, 0, 1 + (w - 1) * 7);
var dow = simple.getDay();
var ISOweekStart = simple;
if (dow <= 4)
ISOweekStart.setDate(simple.getDate() - simple.getDay() + 1);
else
ISOweekStart.setDate(simple.getDate() + 8 - simple.getDay());
return ISOweekStart;
}
``````

Result: the 20th week of 2013 is May 13, which can be confirmed here.

``````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);
``````