Many casual users of e.g. our history pages are confused by timestamps that are given exclusively in UTC without note. Instead of forcing them to guess, we should give them a brief note in the exact same way that we do for timestamps as given in user signatures, with the added tweak of an on-hover/on-click JS tool to explain what UTC is and how it maps to what they think of as time.
This would mean that times shown in article histories, user contribution lists and other places would show as "14:11, 12 January 2015" would instead show as "14:11, 12 January 2015 (UTC)".
For the current edge case of logged-in users with a non-UTC offset preference (T91261: Auto-adjust timezone for anonymous users may impact this), we should equally label those as non-UTC. This would mean that in the above situation instead of "09:11, 12 January 2015" it would show as "09:11, 12 January 2015 (EST)".