[rezonant]

That's all fine, but the problem case as described in TFA is that if the meeting was scheduled for 9AM (perhaps say 06:00 UTC) before the change, but 9AM now means 07:00 UTC, then you can convert that previously saved timestamp back to local time, but it will still be 8AM, which is not when the meeting was scheduled. The humans all arrive at 9AM (an hour later than originally scheduled with respect to UTC, but correct by the clocks on the wall), the software thinks it started an hour ago.