Jquery UI Datepicker with EOM and MOM

I need implement a Jquery UI Calendar that when a users click any date, it select automatically to the next MOM or EOM.

For example, if the user clicks on April 9th, it must go to 'April 15th', or if clicks on 22 February, it must go to 'Feb 28' or 'Feb 29'.

For clarification: MOM is "Middle of Month, every time is 15th", from 1 to 15 of any month. EOM is "End of Month, it's the last day of the month, jan 31, feb 28 or 29, mar 31, apr 30, may 31, jun 30, july 31, aug 31, sep 30, oct 31, nov 30, dec 31...

Thanks

The code that works with IE6/IE7/IE8/IE9 and FF, Chrome, etc.

    <script type="text/javascript">
    $(function() {
        $("#payperiodcalendar input").datepicker({
            dateFormat: 'dd-M-yy',
            onSelect: function(dateText, inst) {
                var selectedDay = inst.currentDay,
                selectedMonth = inst.currentMonth,
                selectedYear = inst.currentYear;



                if (selectedDay <= 15) {
                    var now = new Date(selectedYear, selectedMonth, 15);
                } else {
                    var now = new Date(selectedYear, selectedMonth, LastDayOfMonth(selectedYear, selectedMonth));
                }
                $(this).attr("value", now.format("dd-MMM-yyyy"));

            }
        });
        function LastDayOfMonth(Year, Month) {
            Month++;
            return (new Date((new Date(Year, Month, 1)) - 1)).getDate();
        }
    });



</script>