user2369009 user2369009 - 1 year ago 70
Javascript Question

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);
}

Answer Source
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.

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