the format of the date must be yyyy-mm-dd, you cannot use yy-mm-dd or any other format.
in some cases i have seen form elements with out a value attribute not work correctly, try changing:
<input id="datepicker_1" name="GrBirthDay" type="text" />
<input id="datepicker_1" name="GrBirthDay" type="text" value="" />
if you are still having a problem after changing the format and adding the blank value, send a copy of your page in a zip archive so I can see the full code in context.