That doesn't help if you don't save the time zone rules that were in effect when you converted the original timestamp to UTC. The problem is that you've already

That doesn't help if you don't save the time zone rules that were in effect when you converted the original timestamp to UTC. The problem is that you've already converted to UTC using the wrong rules, so to make things right you need to unconvert then reconvert again. But there's no way to know which timestamps need correcting and which ones don't. The databases keep track of the zone changes so you can use the proper rules for conversions in the past. They don't protect you from changes in the future.