I have this jQuery datepicker code that initially set the minDate of the datepicker. After a user selects a starting date, I used the option attribute to change the minDate of the datepicker, but nothing happens. What did I do wrong and how should I make it work?

Datepicker init code:

$(function () {
    $('#starttime,#endtime,#validity').datepicker({
                numberOfMonths: [1, 2],
                buttonImageOnly: true,
                showAnim: 'slideDown',
                buttonImage: '../images/cal.gif',
                beforeShow: customRange,
                dateFormat: "dd-mm-yy",
                firstDay: 1,
                changeFirstDay: false
    });
});

Datepicker change minimum date code:

var startdate = new Date(2010,4-1,
    4-1,0,0,0);

$("#endtime").datepicker('option','minDate',startdate);