Editing WoW:API GetGameTime (section)

=== Inter-zone and instance issues ===
This function can unexpectedly return results that are inconsistent with actual time for the login realm sever.

The value returned is from the physical instance server you are actually playing on, and not that of the world instance server (realm server) you log into. Servers for instances such as for Raids and PvP are often shared between login world servers. And instance servers are not always running using the same time zone as the login realm server. This is particularly noticeable for Oceanic and other low population world servers.

So for example, if you log into US Ysera EST at 18:00, and enter Ulduar running on an instance server that is PST. Suddenly the time returned will be 3 hours behind the time you were getting.  Confusing but important :)  Please keep this in mind if using this time for anything important functionally, such as diffing times or using as part of functional historical data.

A second compounding issue is that even if the current zone server is in the same time zone as the login server, you might have variance between the two blizzard server's clocks, like 5-10 minutes, though not typically the case.