The problem is that the result of toUTCTimestamp(datetime: January 1st 2077 1:15am, timezone: New York) may be X today, but timezone rules could change soon, and then it becomes Y tomorrow. If you've only persisted (unixTime: X, timezone: New York) in the database and you try to turn X back into a local time, maybe now you get December 31, 11:45pm. If you're a calendar application or storing future times for human events, people will think your software is broken and buggy when this happens, and they will miss events.