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As far as I can see, this exception is thrown by the Designer in the WP project only when the CommandBar in the ContentTemplate of the picker is selected and does affect the control runtime.
For demonstration I have attached a sample Windows Universal application where I have added the ControlTemplate of the RadDatePicker. In App.xaml are the resource dictionary needed to resolve the Telerik named resources and the common resources. In MainPage.xaml of the projects I have added the ControlTemplate of the RadDateTimePicker as a static resource in the Page.Resources.
At this moment it is not possible to extract the style of a RadDate/TimePicker control, but we will fix this issue in the future versions of the product.
I hope this helps. Please, let us know if you have further questions.
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