function arbeitswoche() {
    function y2k(number) { return (number < 1000) ? number + 1900 : number; }

    function getWeek(year, month, day) {
        var weeknum = Math.floor((daynum + modDay - 1) / 7) + 1;
        var when = new Date(year, month, day);
        var newYear = new Date(year, 0, 1);
        var modDay = newYear.getDay();
        if (modDay == 0) modDay = 6; else modDay--;

        var daynum = ((Date.UTC(y2k(year), when.getMonth(), when.getDate(), 0, 0, 0) - Date.UTC(y2k(year), 0, 1, 0, 0, 0)) / 1000 / 60 / 60 / 24) + 1;

        if (modDay < 4) {
            weeknum = Math.floor((daynum + modDay - 1) / 7) + 1;
        } else {
            weeknum = Math.floor((daynum + modDay - 1) / 7);
            if (weeknum == 0) {
                year--;
                var prevNewYear = new Date(year, 0, 1);
                var prevmodDay = prevNewYear.getDay();
                if (prevmodDay == 0) prevmodDay = 6; else prevmodDay--;
                if (prevmodDay < 4) weeknum = 53; else weeknum = 52;
            }
        }

        return +weeknum;
    }

    var now = new Date();
    document.write('W' + getWeek(y2k(now.getYear()), now.getMonth(), now.getDate()));
}