Thanks, that worked great,l Kaloyan, however, I needed to make a couple of changes.
First, I assume you meant to say KeyDown, not KeyUp, because that is in the name of your event handler.
Second, because the time picker is in a CellEditTemplate, I couldn't use the picker name directly. It wasn't recognized, so I casted the sender to a time picker to get the object.
Third, I found I had to set e.Handled = true to avoid getting the plus or minus added in the time picker field.
In case anyone is interested, here is the revised code: