I had a similar issue when syncing a bunch of Gmail emails onto my desktop. Somehow I screwed up the dates on the messages (even though the headers are the same), and now my messages are in a weird random order, and finding stuff based on date is impossible.
Given that it is roughly 15 years' worth of emails, this is … not great.
Given that it is roughly 15 years' worth of emails, this is … not great.