Kendo UI Calendar - clears past dates when calendar icon clicked - date

Question

In my screen the Kendo UI Calendar used behaves odd.
When i come on to editing the screen and click on Calendar field if the date is past date (disabled on calendar) then the existing date value gets cleared, even though i am not making any change.
How can i persist the date value for the past dates that are disabled in the calendar.
It would be helpful to give sample code for the fix.
The same we tried with HTML calendar and JQuery Calendar and the behavior is same as of Kendo UI Calendar.
The code used is in javascript
$('#txtWFITaskStartDate').change(function (e) {
e.preventDefault();
e.stopImmediatePropagation();
var labelName = $(this).data('validatelabel');
var CurrentId = $(this).attr('id');
var startDate = $("#" + CurrentId).val();
var duration = $('#txtWFITaskDuration').val();
if ($('#' + CurrentId).val() === "") {
varErrorClassName = 'errmsgStartDate';
DisableTaskEditControls();
$('#StartDate').after('<span class="text-danger ' + varErrorClassName + '">' + Web_IsRequired.replace("{0}", labelName) + '</span>');
}
else {
$('.errmsgStartDate').remove();
$('.errmsgEndDate').remove();
EnableTaskEditControls();
var endDate = CalculateEndDate(duration, startDate);
$('#txtWFITaskEndDate').val(endDate);
$('#EndDate').datepicker('setDate', new Date(endDate));
endDate = $('#txtWFITaskEndDate').val();
if (endDate !== "") {
EnableTaskEditControls();
} else {
varErrorClassName = 'errmsgEndDate';
$('#EndDate').after('<span class="text-danger ' + varErrorClassName + '">' + Web_IsRequired.replace("{0}", "End date") + '</span>');
DisableTaskEditControls();
}
}
});
Below is the screenshot of the dates displayed on modal popup view (MVC)
As the Start Date is past date the control displays in disabled mode. When focus to the Start Date control and focus out from it. The past date gets cleared. I want to hold the existing value, until user changes the date else the same value has to persist in the Start date text box.