function ChangeDayIfNecessary( form )
{
    if (form.day.options.length < 29)
    {
        NeuerEintrag = new Option("29", "29", false, false);
        form.day.options[28] = NeuerEintrag;
    }
    if (form.day.options.length < 30)
    {
        NeuerEintrag = new Option("30", "30", false, false);
        form.day.options[29] = NeuerEintrag;
    }

    if (form.day.options.length < 31)
    {
        NeuerEintrag = new Option("31", "31", false, false);
        form.day.options[30] = NeuerEintrag;
    }

    if (form.month.value == "2")
    {
        form.day.options[30] = null;
        form.day.options[29] = null;
        if (!Schaltjahr(form.year.value))
        {
            form.day.options[28] = null;
        }
    }
    else
    {
        if (form.month.value == "4" || form.month.value == "6" || form.month.value == "9" || form.month.value == "11")
        {
            form.day.options[30] = null;
        }
    }
}

function Schaltjahr(Jhr)
{
    Jahr = (Jhr >= 2000) ? Jhr : ((Jhr< 80) ? Jhr + 2000 : Jhr + 1900) 
    SJahr = Jahr%4; 
    SHJahr = Jahr%100; 
    S4Jahr = Jahr%400; 
    STag = ((S4Jahr == "0") ? (1) : ((SHJahr == "0") ? (0) : ((SJahr == "0") ? (1) : (0))));
    
    return STag;
}