Feb 02, 2015 07:54 AM|anuj_koundal|LINK
Hi parth I have created a demo application according to your code and it is working fine at my end. Might be ur css is conflicting with another properties. I your site is live post a link.
or try this solution: http://stackoverflow.com/questions/8591014/calendar-extender-in-modal-popup-extender-asp-net-not-working