I should also clarify that conceptually, even if you keep your copy of the tzinfo database obsessively up-to-date, you could still experience what appear to be

I should also clarify that conceptually, even if you keep your copy of the tzinfo database obsessively up-to-date, you could still experience what appear to be retroactive updates, per the "Theory and Pragmatics" document from the tzinfo project [1]. That's because some governments are pretty sloppy about announcing their timezone changes in advance. It's compounded by the tzinfo DB being maintained by volunteers (who can't spend their lives obsessively gathering timezone change announcements the instant they're made), and the lead time necessary for software projects to update their tzinfo instances [2]. So, while retroactive changes may not be consciously made often or ever, it's certainly an effect that happens in practice. 1: https://data.iana.org/time-zones/theory.html#accuracy https://data.iana.org/time-zones/theory.html#accuracy 2: https://codeofmatt.com/on-the-timing-of-time-zone-changes/ https://codeofmatt.com/on-the-timing-of-time-zone-changes/