function catcalc(cal) {
	var date = cal.date;
	var time = date.getTime()
	// use the _other_ field
	var field = document.getElementById("CHECK_OUT_DATE");
	if (field == cal.params.inputField) {
		time == field; // substract one week
	} else {
		time += Date.DAY; // add one week
	}
	var date2 = new Date(time);
	field.value = date2.print("%Y-%m-%d");
}
Calendar.setup({
	inputField: "CHECK_IN_DATE",
	ifFormat: "%Y-%m-%d",
	showsTime: false,
	timeFormat: "24",
	onUpdate: catcalc
});
Calendar.setup({
	inputField: "CHECK_OUT_DATE",
	ifFormat: "%Y-%m-%d",
	showsTime: false,
	timeFormat: "24",
	onUpdate: catcalc
});