datepicker，结束时间必须大于开始时间

$('#js-start-time').datepicker({
dateFormat:'yy-mm-dd',
onSelect: function( startDate ) {
var $startDate = $( "#js-start-time" );
var $endDate = $('#js-end-time');
var endDate = $endDate.datepicker( 'getDate' );
if(endDate < startDate){
$endDate.datepicker('setDate', startDate - 3600*1000*24);
}
$endDate.datepicker( "option", "minDate", startDate );
}
});
$('#js-end-time').datepicker({
dateFormat:'yy-mm-dd',
onSelect: function( endDate ) {
var $startDate = $( "#js-start-time" );
var $endDate = $('#js-end-time');
var startDate = $startDate.datepicker( "getDate" );
if(endDate < startDate){
$startDate.datepicker('setDate', startDate + 3600*1000*24);
}
$startDate.datepicker( "option", "maxDate", endDate );
}
});