How should I compare GA4 reports across a timezone change without misreading the spike or missing hours?

Treat the timezone-change date as a reporting boundary, not a normal traffic spike or drop. GA4 uses the property timezone to set each report day, and changing it only affects data going forward, so the switch can leave one oddly short or long reporting day; reports may also briefly use the old timezone while Google processes the change. For a clean comparison, compare completed days before the switch in the old timezone with completed days after it in the new one, and don’t use day-over-day or hourly totals that straddle the change. If you need an exact reconciliation, use event-level data from a BigQuery export and regroup it in one chosen timezone; GA4’s standard reports won’t retroactively recut the older dates. Property change history is handy for confirming the exact time of the switch.