Trying to return javascript date based on "week 4 2013" using date.js

Question

I'm trying to figure out the date from a week number (date would be week starting on [date]).

It works on the home page of the date.js website (see image), but I can't get it to work.

I'm using var date = Date.parse('week 5 2013');, but that always returns today's date.

What am I doing wrong?

Thanks, Angus

Answer 1

If you are not wedded to date.js it is pretty simple to calculate the nth week of a year-

The first week of a year begins on the first Monday of that year,

if the first week includes a thursday.

Otherwise the first week of a year (like 2013)

begins on the last Monday of the previous year.

function nthWeek(y, n){
    if(y== undefined) y= new Date().getFullYear();
    var start= new Date(y, 0, 1);
    if(start.getDay()>4) start.setDate(start.getDate()+7);
    while(start.getDay()!= 1) start.setDate(start.getDate()-1);
    if(!n || n== 1) return start;
    start.setDate(start.getDate()+((n-1)*7));
    return start;
}




var s='week 4 2013';
var m=s.match(/(\d+)/g);

alert(nthWeek(m[1],m[0]));

/* returned value: (Date) Mon Jan 21 2013 00:00:00 */