Why Were My Timezone Definitions Updated? 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?

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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.