[рони]

Вариант )))

function DateOfMondayInWeek(b, c) {
    for (var a = 1;; a++) if ((new Date(b, 0, a)).getDay() == 1) break;
    a += (c - 1) * 7;
    return new Date(b, 0, a)
}
alert(DateOfMondayInWeek(2010, 1));