alt Hacker NewsCorrect according to what? An employee who punched in at 9AM wouldn't show up as having punched in at 9AM anymore. Not unless you also store the exact timezone the UTC timestamps have been created with - but that's basically local timestamps with extra steps.
tzdata doesn’t change retroactively [1].
If an employee clocks in at 2026-06-22 09:00 America/Sao_Paulo time, (which has a -03:00 offset today), and the server's clock is in UTC, the server will save 2026-06-22 12:00 to the database.
If America/Sao_Paulo changes to -02:00 on 2027, it doesn’t affect conversions for past dates. You still get 2026-06-22 09:00 when trying to convert 2026-06-22 12:00 to local time in America/Sao_Paulo.
edit: [1] unless it was wrong. in which case, you actually still want the UTC timestamp stored, so that you can just update tzdata and get correct local times, as opposed to saving the wrong local times in your database, that you now have to also fix.