Caverns of Time: Escape From Durnhold Keep Dec 11 2010
The first instance in the Caverns of Time, Old Hillsbrad has you travelling back in time to rescue Thrall and stop the Infinite Dragonflight from altering history. The Caverns of Time are located in the eastern part of Tanaris, but there is also a portal in Dalaran that will take you there.
This is a level 66-68 dungeon. The final boss is Epoch Hunter.