How to Disable Past Date Using Datepicker using JQuery

<script src="jquery.min_1.9.1.js"></script>
<script src="bootstrap-datepicker.js"></script>
<link rel="stylesheet" type="text/css" href="bootstrap-datepicker.css" />
<div class="form-group">
<input id="dp1" type="text" class="form-control input-sm" id="DtChkIn" placeholder="Data CheckIn" >
</div>
</div>
<div class="form-group">
<input id="dp2" type="text" class="form-control input-sm" id="DtChkOut" placeholder="Data CheckOut" >
</div>
</div>
<script>
var nowTemp = new Date();
var now = new Date(nowTemp.getFullYear(), nowTemp.getMonth(), nowTemp.getDate(), 0, 0, 0, 0);
var checkin = $('#dp1').datepicker({
beforeShowDay: function (date) {
return date.valueOf() >= now.valueOf();
},
autoclose: true
}).on('changeDate', function (ev) {
if (ev.date.valueOf() > checkout.datepicker("getDate").valueOf() || !checkout.datepicker("getDate").valueOf()) {
var newDate = new Date(ev.date);
newDate.setDate(newDate.getDate() + 1);
checkout.datepicker("update", newDate);
}
$('#dp2')[0].focus();
});
var checkout = $('#dp2').datepicker({
beforeShowDay: function (date) {
if (!checkin.datepicker("getDate").valueOf()) {
return date.valueOf() >= new Date().valueOf();
} else {
return date.valueOf() > checkin.datepicker("getDate").valueOf();
}
},
autoclose: true
}).on('changeDate', function (ev) {});
</script>