> "It's 1900 UTC, X is awake from....err....2200 to 1400, so I can call now"

Assuming it's that simple of course. Like, you can do mental math about what UTC "officially means" for someone many miles away, but people coordinate with others even if it means their local schedule is not aligned with that ideal case. Time zones account for this by being wider or narrower than their Platonic 15° of longitude ideal in some places.