Out of curiosity, what is the use case for this? You should just have to bind data to the datetime picker widget, and not bind the datetime picker widget DateTime target to anything. Does that make sense?
This option doesn't exist, but you can use a value display widget to show ReadOnly DateTime, and just change visibility between Value Display and DateTimePicker depending on ReadOnly state.
I am not sure about your exact use case but one way to achieve read-only date time is to map DateTimePicker widget output to a TextBox and make TextBox property marked as "ReadOnly".