We use a single time zone for our announcements so that everyone is on the same page. Europe spans 4 time zones, Asia spans 8 time zones, North America 5 time zones, etc. We keep everything in one time listing so that the relative position of server openings is clear.
Since the servers being merged are always in the same timezone would it not make more sense to do it in server time? That is what the players are used to.
The opinions expressed on this post are my own as a player, not as a Moderator.