How to add Days in a islamic or HijriDate?

I have two text boxes.One is for taking StartDate from a islamic calender,so i am using jQuery Calendars Datepicker. The ExpireDate textbox will automatically 60days from StartDate.so i am writing the logic in Onclose event of datepicker.

$(document).ready(function () {
    ShowCalender();
});
function ShowCalender() {
    var calendar = $.calendars.instance('islamic');
    $('[id$=TxtOrderDate]').calendarsPicker({
        calendar: calendar,
        onClose: function (dates) {
            var expiryDate = new Date(dates);
            var x = 60;
            expiryDate.setDate(expiryDate.getDate() + x);               
            document.getElementById('<%=TxtExpirationDate.ClientID%>').value = expiryDate;
        },
        showTrigger: '<img src="../../../_layouts/15/1033/Saudia/Images/calender.png" alt="Popup" class="trigger img">'

    });
}

but here the ExpireDate fill with Date Sun Apr 24 03:00:00 UTC+0300 1436,which is a gregorian date. expiredate mustbe the islamic Date.

How to add specific number of Days in FromDate? Please help.