You can use method getDate():
$('#calendar').datepicker({
dateFormat: 'yy-m-d',
inline: true,
onSelect: function(dateText, inst) {
var date = $(this).datepicker('getDate'),
day = date.getDate(),
month = date.getMonth() + 1,
year = date.getFullYear();
alert(day + '-' + month + '-' + year);
}
});