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There seems to be a bug in the RadDate/TimePicker control when the height of the screen exceeds a certain value. I have redirected this issue to our development team. As a workaround you can increase the height of the selector header, which will decrease the available space for the picker lists. I have updated your Telerik points as a token of gratitude for bringing this issue to our attention.
The list items could be customized with a style that targets the DateTimeListItem type. This style can be included as an implicit resource in the RadTimeSelector control in the Template of the RadTimePicker. I have attached a sample project with a RadTimePicker and its style including the implicit DateTimeListItem Style.
Unfortunately, you can not change the source of the picker lists.
I hope this helps. Please, let us know if you have further questions.
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