function SetDayNames(daySelect, monthSelect, datum) {
	alert('month')
    	var month    = GetValue(monthSelect).substring(0,2);
    	var year     = GetValue(monthSelect).substring(3,7);
    	var day      = 1;
    	var monthDay = new Date(year, month-1, day, 0, 0, 0);
    	var oldMonth = monthDay.getMonth();
    	var names    = 'zondag,maandag,dinsdag,woensdag,donderdag,vrijdag,zaterdag'.split(',');
		if (datum == 1) {
			if ((GetForm().ctl00$ContentPlaceHolder1$DropDownListmaandjaar1.selectedIndex + 1) == GetForm().ctl00$ContentPlaceHolder1$DropDownListmaandjaar1.length) {
				var max_days_future = 5;
			} else {
				var max_days_future = 31;
			}
		} else {
			if ((GetForm().ctl00$ContentPlaceHolder1$DropDownListmaandjaar2.selectedIndex + 1) == GetForm().ctl00$ContentPlaceHolder1$DropDownListmaandjaar2.length) {
				var max_days_future = 5;
			} else {
				var max_days_future = 31;
			}
		}
		
    	daySelect.length = max_days_future;
    	for (day=1; day<=max_days_future; day++) {
        	monthDay.setDate(day);
        	if (oldMonth != monthDay.getMonth()) {
            	// This month has less then 31 days 
            	// because the next day is already in the next month
            	daySelect.length = day-1;
            	break;
        	}
        	daySelect.options[day-1].value = day;
        	daySelect.options[day-1].text  = names[monthDay.getDay()]+" "+day;
    	}
	}

	function SetWeekDayNames() {
    	SetDayNames(GetForm().ctl00$ContentPlaceHolder1$DropDownListdag1, GetForm().ctl00$ContentPlaceHolder1$DropDownListmaandjaar1, 1);
    	SetDayNames(GetForm().ctl00$ContentPlaceHolder1$DropDownListdag2, GetForm().ctl00$ContentPlaceHolder1$DropDownListmaandjaar2, 2);
	}
	
	function AutoPeriod(){
		var f      = GetForm();
		var jaar   = GetValue(f.maandjaar1).substring(3,7);
		var maand  = GetValue(f.maandjaar1).substring(0,2);
		var dag    = f.dag1.selectedIndex+1;
		var datum1 = new Date(jaar, maand-1, dag, 0, 0, 0, 0);
		var datum2 = new Date(datum1.valueOf() + (7*24*3600*1000)); //Add 14 days (in msec)
		
		jaar = datum2.getYear();
		maand = datum2.getMonth() + 1;
		if (maand < 10){
			maand = "0" + maand;
		}
		
		if ((f.maandjaar1.selectedIndex + 1) == f.maandjaar1.length && (datum2.getDate() + 1) > f.dag1[f.dag1.length - 1].value) {
			SetValue(f.maandjaar2, maand + "/" + jaar);
			SetWeekDayNames();
			SetValue(f.dag2, f.dag2[f.dag1.length - 1].value);
		} else {
			SetValue(f.maandjaar2, maand + "/" + jaar);
			SetWeekDayNames();
			SetValue(f.dag2, datum2.getDate());
		}
		return true;
	}