Hello Harry,
This seems to be an issue with old versions of RadDatePicker. You could either upgrade to latest version of RadDatePicker where this does not happen, or you can refer to the code library for explicitly showing the popup above the DateInput.
How to open RadDatePicker calendar depending on its position
Also here is the KB article which will assist you in upgrading to new version:
Updating RadControls for ASP.NET to another version or license
-Shinu.