Florian:
I copied over your code and ran it. By simply changing the property setting for the RadDatePicker to "Width="100px" the full date displays correctly. I believe that this width setting takes in the entire width of the component, textbox and popup calendar combined.
Default.aspx:
Alternatively, you can remove the width setting from the RadDatePicker definition and set it as a property of the dateinput textbox:
Default.aspx:
The latter works better and the "30px" assignment displays the full date.
Regards,