Editing WoW:API GetGameTime (section)

=== Compensating for inter-server differences ===
However... with the newer problem, where the servers themselves may have different server times even though they are accessible from the same realm login, (and the original reason for the extended note) you will need to basically shut off the base correction when a player enters an instance and save the difference value to your saved vars, in case the player restarts or logs into an instance instead of the realm world server.

This leaves the problem of a user installing the AddOn and logging on to an instance server first rather than the realm world server (and people who raid know that can really happen a lot, ala "oh let me go enable or install that AddOn I need, brb"). In this case you will need to seed the base with the difference of the instance server just once and then follow the rule of shutting it off in an instance, where it will naturally be corrected within a minute or so of entering realm world server.

This is a lot of trouble to get simple consistent server time, and ... may seem crazy to worry about. For some apps is more than nice to have, ala player joins and leaves for say raid tracker for example and dealing with non-trivial things for users to fix after the fact like wtf DKP upload results, its worth it. The core issue isn't having accuracy like an atomic clock, but not having time values jump in a way that makes the data absurd or inconsistent or impossible to process like DKP uploads, and a close second is all users having the basic same time of the day in a consistent format and time zone, which is really the same thing for multi user uploads in my example.  All Blizzard could do to fix this would be to make a hole where to continue to get server time from the realm server for AddOns and possibly share the seconds counter as a third return value in this API function if there is such a thing, and is all probably not a trivial thing to change.

Lastly, as an aside, the suspicion is that the server to client time updates are actually done in a low bandwidth fashion in one of several ways. They are taken directly from other data for other purposes like timings from one of the standard server game clocks out of packets intended for avatar updates and the like with the seconds clipped off. Another possibility could be a very occasional update from the server, out of band, and updated by the client based on local time anyway. If is out of band and special the last example would be the least bandwidth by far. Regardless it would have been nice to have an extra return value with the seconds from GetServerTime, with wherever the client's value used to know when to flip the seconds value, rather than re-synthesize it again on the Lua side. :)