There’s actually a couple of threads about this. Realm connections caused time zone changes and there was no notice about it.
Our server Ghostlands/Kaelthas had multiple servers merged into ours recently and as soon as that happened, we went from CST to EST, with no notice at all.
Announcement thread: Realm Connections -- September 17-18 (notice no mention of the timezone being changed on us).
I’ve asked on Twitter for clarification on if this is a bug or intended, but no reply so far.