Thank you for contacting Telerik support.
First I want to say that we have a reported issue with RadDateTimePicker where the NullDate and MinDate are set to the same date. You can track the item for status changes and add your vote for it here.
Also in order to display the null text when the control loses focus you just have to set the NullDate to equals the Value date. Basically when this two dates are equal the control will display the null text:
radDateTimePicker1.NullDate = radDateTimePicker1.Value;
I hope this helps. Should you have any other questions do not hesitate to ask.
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