Would it be possible to avoid any ambiguity at all by saving the timezone rules themselves as separate most-previous and current versions within the application context itself?

Then when a new set of rules is detected (perhaps notification of updates at the OS level should be turned into a standardized publish-subscribe API to avoid too many applications polling for changes all the time, especially wasteful on battery-powered devices), move the current version to the previous version, scan for timezone rule differences, then scan your collection of records that use those timezones, parse them down to only those records affected by the new rules, and finally apply corrections to those records affected by the new rules.

Extending further, if this approach works, perhaps an OS-level, perhaps git-backed storage of versions of timezone rules could avoid duplication of effort. Combined with a publish-subscribe service, it can auto-prune itself based upon the list of subscribed applications and when the oldest subscriber last hit the version store.