Use the javascript Date object.
$(document).ready(function() {
$('#calendar').datepicker({
dateFormat: 'yy-m-d',
inline: true,
onSelect: function(dateText, inst) {
var date = new Date(dateText);
// change date.GetDay() to date.GetDate()
alert(date.getDate() + date.getMonth() + date.getFullYear());
}
});
});