Hello Ingo,
Thank you for writing.
I have logged it in our feedback portal and I have added a vote for it on your behalf. You can track its progress, subscribe for status changes and add your comments on the following link - feedback item.
I have also updated your Telerik points.
Currently, the possible solution that I can suggest is to use the RadDateTimePickerCalendar.Calendar.ElementRender event and disable the cells that are not in the applied interval:
I hope this information helps. Should you have further questions I would be glad to help.
Regards,
Dess
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