jquery-ui/demos/functional/templates/ui.datepicker.two.html

<input type="text" size="10" value="" id="startDate"/> to <input type="text" size="10" value="" id="endDate"/>
<script type="text/javascript">
function customRange(input) {
	return {minDate: (input.id == "endDate" ? $("#startDate").datepicker("getDate") : null),
		maxDate: (input.id == "startDate" ? $("#endDate").datepicker("getDate") : null)};
}
</script>