RadDatePicker enable/disable through javascript

Problem: How to enable/disable RadDatePicker through javascript ?

Solution: You can make use of the client side method set_enabled to false.

Example:

function disable()
  {
      var dateinput = $find("<%= RadDatePicker1.ClientID %>");
      dateinput.set_enabled(false);
  }

Also refer the following documentation for more on the RadDatePicker Client Object.
RadDatePicker Client Object