We have all kinds of low-level protocols for coordinating all manner of things. Distilling a coherent system of agreeing on a timezone difference, between two communicating entities and presenting that to the user at any time would be quite basic.
We have location data for all, at all times. We have a local, accurate clock. There's little else we'd need. What remaining issues are all UX. Apple could roll such a thing out parallel on all their multitude devices and it'd be basically trivial.
Is it maybe just habit that we gravitate to a manual process, when an automated one would be rather easy in this situation?
On iOS, the existing clock app already provides a basic form, tapping the plus button allows the addition of multiple zones and provides useful additional context in the form of offsets and what day it is relative to the users local date and time.
That's halfway there, it would only need contact awareness to be able to provide a solution.