Hi,
Not long ago, someone contributed an enhancement to the DatePicker control which is a Property called CalendarSelectionOnly which if true disables the textbox so it can ony be updated by the control. However, I didn't notice that the default was true for this new property. I have changed the default to flase here and will commit to svn later today. If you are working with the release version, you can edit the Admin/ContentManager.aspx page with a text editor. Find the dpEndDate control within the GridView and add a property CalendarSelectionOnly="false"
Then you will be able to click in the textbox to clear the end date.
Thanks,
Joe