http://weeknumber.net/how-to/javascript How to get the week number from a date

The code below adds two new functions to the Date object. Add this to your source code.

To get the ISO week number (1-53) of a Date object, mydate, use mydate.getWeek().

To get the corresponding four-digit year, use mydate.getWeekYear().

// This script is released to the public domain and may be used, modified and distributed without restrictions. 

// Attribution not necessary but appreciated. 
// Source: http://weeknumber.net/how-to/javascript 
// Returns the ISO week of the date. 

Date.prototype.getWeek = function() { 
    var date = new Date(this.getTime()); 
    date.setHours(0, 0, 0, 0); 
    // Thursday in current week decides the year. 
    date.setDate(date.getDate() + 3 - (date.getDay() + 6) % 7); 
    // January 4 is always in week 1. 
    var week1 = new Date(date.getFullYear(), 0, 4); 
    // Adjust to Thursday in week 1 and count number of weeks from date to week1. 
    return 1 + Math.round(((date.getTime() - week1.getTime()) / 86400000 - 3 + (week1.getDay() + 6) % 7) / 7); } 

// Returns the four-digit year corresponding to the ISO week of the date. 
Date.prototype.getWeekYear = function() { 
    var date = new Date(this.getTime()); 
    date.setDate(date.getDate() + 3 - (date.getDay() + 6) % 7); 
    return date.getFullYear(); 
}