I just got this update notification

Update time zones
New time zone definitions are available. Time zones will be updated the next time your phone restarts.

Did any real time zones actually change or was this a bug fix? What changed?

[Time Zone Update Notification[1]

  • Usually the time zones change because one country has changed their daylight saving time start/end date. Of course other changes are possible but happen very seldom. – Robert May 19 at 15:22
  • 3
    Easily found through a simple Google Search. The Kingdom of Morocco changed it DST initiation date to May 31, 2020 where currently it was established for May 24. The update establishes the new rule. – wbogacz May 19 at 15:44

See Android source for Time Zone Rules

Thanks to wbogacz who pointed out in comments the latest change (Morocco) which is likely the cause of this update or the IANA update of 23 Apr 2020

Update Mechanism

Time zone updates (Android 8.1–9)

  • APK-based mechanism to push updated time zone rules data to devices without requiring a system update.

  • Android core libraries team provides the necessary data files for updating time zone

  • OEMs can choose to use these data files when creating time zone updates for their devices or can create their own data files

Time zone updates (Android 10+)

Updates use the following process:

  • IANA releases an update to the Time Zone Database releases an update in response to one or more governments changing a time zone rule in their countries.

  • Google or the Android partner prepares a Time Zone Data module update (APEX file) containing the updated time zones.

  • The end-user device downloads the update, reboots, then applies the changes, after which the device's time zone data contains the new time zone data from the update.

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