If you set the TimeInterval of the RadDateTimePicker to a short interval, this will generate a lot of items in the clock area (the right section of the drop down, next to the calendar). If the items have not enough space to draw properly, they will start overlapping (thus clipping) one another. 
Add a way to enable a ScrollViewer for these situations.

Currently, this can be done by re-templating the RadClock control, as shown in the following KB article.