In a project I use RadDateTimePicker (InputMode="DatePicker", DateTimeWatermarkContent="Custom watermark") as a separate control and as a part of DataTemplate for items in ListBox (ItemsControl). Initially I used version 2013.3, then I tried to use version 2015.2. Separate controls work fine. But RadDateTimePicker in DataTemplate began to display "EnterDate" for watermark content. The only thing I changed was Telerik dlls of another version.
I attached screenshots for different versions. Project's .net version - 4.6, Windows 10, 64bit.