Ugenummer når 1/1 er i uge 52/53 og 31/12 er i uge 1

Ok, jeg fandt selv ud af det:

Date.prototype.getMDay = function() {
return (this.getDay() + 6) %7;}
Date.prototype.getISOYear = function() {
var thu = new Date(this.getFullYear(), this.getMonth(), this.getDate()+3-this.getMDay());
return thu.getFullYear();}
Date.prototype.getISOWeek = function() {
var onejan = new Date(this.getISOYear(),0,1);
var wk = Math.ceil((((this - onejan) / 86400000) + onejan.getMDay()+1)/7);
if (onejan.getMDay() > 3) wk--;return wk;}