function catcalc(cal) {
 var date = cal.date;
 var time = date.getTime()
 // use the _other_ field
 var field = document.getElementById("CheckOut");
 if (field == cal.params.inputField) {
 field = document.getElementById("CheckIn");
 time -= Date.DAY; // substract one Day
 } else {
 time += Date.DAY; // add one Day
 }
 var date2 = new Date(time);
 field.value = date2.print("%d.%m.%Y");
 }
 Calendar.setup({
 inputField : "CheckIn", // id of the input field
 ifFormat : "%d.%m.%Y", // format of the input field
 firstDay : 1,
 onUpdate : catcalc
 });
 Calendar.setup({
 inputField : "CheckOut",
 ifFormat : "%d.%m.%Y",
 firstDay : 1
 }); 
 
 <!--
 function CheckSearchParams(form)
 {
 if (form.CheckIn.value == 'tt.mm.jjjj')
 {
 alert('Bitte geben Sie ein gültiges Anreisedatum ein!');
 form.CheckIn.focus();
 return false;
 }

 if (form.CheckOut.value == 'tt.mm.jjjj')
 {
 alert('Bitte geben Sie ein gültiges Abreisedatum ein!');
 form.CheckOut.focus();
 return false;
 }
 return true;
 }
->