How to format date with hours, minutes and seconds when using jQuery UI Datepicker?

$("#datepicker").datepicker("option", "dateFormat", "yy-mm-dd ");

For the time picker, you should add timepicker to Datepicker, and it would be formatted with one equivalent command.

EDIT

Use this one that extend jQuery UI Datepicker. You can pick up both date and time.

http://trentrichardson.com/examples/timepicker/

Try this fiddle

$(function() {
    $('#datepicker').datepicker({
        dateFormat: 'yy-dd-mm',
        onSelect: function(datetext) {
            var d = new Date(); // for now

            var h = d.getHours();
            h = (h < 10) ? ("0" + h) : h ;

            var m = d.getMinutes();
            m = (m < 10) ? ("0" + m) : m ;

            var s = d.getSeconds();
            s = (s < 10) ? ("0" + s) : s ;

            datetext = datetext + " " + h + ":" + m + ":" + s;

            $('#datepicker').val(datetext);
        }
    });
});

Using jQuery UI in combination with the excellent datetimepicker plugin from http://trentrichardson.com/examples/timepicker/

You can specify the dateFormat and timeFormat

$('#datepicker').datetimepicker({
    dateFormat: "yy-mm-dd",
    timeFormat:  "hh:mm:ss"
});