How do I add calender control in dropdownlist

The answer above looks like for Windows control...

For ASP.NET:
Lets say we create a user control that does what you ask.
Drop a textbox, adjacent to it an image (this should give a feel of a dropdown) and then a calendar on the control.

Now, on click of textbox/image:
tie the visibility of the calendar control (also can toggle the image if needed). Once the calendar is visible, select the date and on selection you can hide back the calendar and show the selected date in the textbox (all this can be done on client side)

This in total would give you a feel of a dropdown having calendar.