The in game calendar says that Zul’Gurub and Ruins of Ahn’Qiraj reset 19 hours later than they actually do.
Last night (Monday) I checked the in game calendar and enabled Raid Resets in the filters so I could see when ZG was scheduled to reset. It said that it would reset at 5am on Wednesday. So based on this information I didn’t push to do ZG on Monday and instead did it on Tuesday. After zoning in it clearly states that ZG will reset on Friday at 10AM, which means that it reset today, Tuesday, at 10AM. I did the mount bosses to get saved and checked my /raidinfo and it agreed. The Ruins of Ahn’Qiraj are on the same schedule and have the same bug.
Looks like Zul’Aman is correct though, has the right timing on the in game calendar.