here is a complete code with example:
JS:
var $datepicker = $( "#datepicker" );
$datepicker.datepicker();
var date = new Date();
var newdate = new Date(date);
newdate.setDate(newdate.getDate() - 7);
$datepicker.datepicker('setDate', newdate);
HTML:
<script>
function minus()
{
var $datepicker = $( "#datepicker" );
var date = $datepicker.datepicker('getDate');
var newdate = new Date(date);
newdate.setDate(newdate.getDate() - 1);
$datepicker.datepicker('setDate', newdate);
}
</script>
<input type="text" id="datepicker">
<a href="#" onclick="minus()">minus date</a>
JSFIDDLE:
http://jsfiddle.net/epHNZ/295/