function catcalc(cal) {
		var date = cal.date;
        var time = date.getTime()
        // use the _other_ field
        var field = document.getElementById("tt_ruecktermin");
        if (field == cal.params.inputField) {
            field = document.getElementById("tt_termin");
            time -= Date.WEEK*2; // substract one week
        } else {
            time += Date.WEEK*2; // add one week
        }
        var date2 = new Date(time);
        field.value = date2.print("%d.%m.%Y");
    }