Thank you for writing.
The event is raised by the mask date time provider which is used by the editor of the RadTimePicker
control. This provider is working with CancelEventArgs and that is why such a type is passed as an argument. In the described scenario, the event argument which is created is actually ValueChangingEventArgs
and you can cast it in the event handler in your form:
sender, CancelEventArgs e)
ValueChangingEventArgs args = e
I hope this helps. Let me know if you need further assistance.