Hi!
I think it is currently possible to do this on the Android platform using the 'now()' function, although it is somewhat limited by the declarative nature of XForms, so there may be edge cases where it doesn't work.
There's a forum thread discussing the topic and an implementation here: https://forum.dimagi.com/t/once-function-from-odk-in-commcare/7913/2
Let me know if you have any questions or issues implementing this approach.
-Clayton