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For the rest of your statement: for times not tied to a particular location or time offset, you should always use UTC in the form of the offset Z, in ISO 8601/RFC 3339 terms, since specifying any other offset indicates that it means something. (Note that RFC 3339 tried to have -00:00 be the neutral offset and Z and +00:00 meaningful, but that is acknowledged to have failed, and so https://www.ietf.org/archive/id/draft-ietf-sedate-datetime-e... is updating it to match actual usage.) But for things that involve humans and are anchored to a particular time zone, using UTC and not storing a time zone is wrong: you should store the relevant time zone and (fallback) offset so that if the time zone definition changes (as they do, sometimes with less than a few days’ notice), future times can be corrected, which they can’t be if you anchored them to UTC. So: things like system logs, use UTC; online conferences, use UTC; location-bound conferences, use that location’s time zone; general user calendars, use the user’s time zone; calendars for companies that straddle time zones (or people that work across time zones): deliberately choose a time zone or offset to anchor things to (sometimes at the level of individual events), especially for the sake of recurring event periods if you use a time zone with DST.

While a datetime without a timezone isn't terribly useful, distinguishing it from a datetime and timezone designation pair is the only correct type system.