A silent error is printed in the Output pane of Visual Studio at application startup. The error doesn't cause any issues with the control. The application starts normally and the RadDateTimePicker control works as expected.
The error message is the following:
System.Windows.Data Error: 4 : Cannot find source for binding with reference 'ElementName=TodayButtonBorder'. BindingExpression:Path=CornerRadius; DataItem=null; target element is 'RadButton' (Name='TodayButton'); target property is 'CornerRadius' (type 'CornerRadius')