From World of Warcraft's site,
"At the heart of the Tanaris desert lie the Caverns of Time, home of the bronze dragonflight. Nozdormu and his flight work to safeguard the time stream against all who might disturb the delicate pathways of fate. Lately a malevolent force has infiltrated the timeways, and the efforts of this mysterious enemy could lead to a destabilization of the world, and even time itself."
The Keepers of Time are the dragons living in Tanaris's Caverns of Time - to start getting reputation with them, make sure to first talk to the Dragon NPC infront of the cave, and take the ride down with him to meet his Master, Andormu. Once you do so, you will need to finish a small quest that will make you take a tour around the inner cave, showing you the two instances you can access, as well as the entrance to the battle of Hyjal should you complete the quest for the Scale of the Sands's envoy.
The two instances in the Caverns of Time are the only places where you may earn Keepers of Time reputation - first time through you will obtain a massive amount of reputation completing the quests you get, subsequent runs will only yield reputation through killing monsters within Old Hillsbrad and the Black Morass.
Keep in mind the Scale of the Sands is an entirely different Faction, that yields an epic ring much like the former Brood of Nozdormu back when AQ40 was out - you may upgrade your ring if you've completed their quests, the first of which involves killing Lady Vashj (SerpentShrine Cavern's Last Boss) and Prince Kael'thas (The Eye's Last Boss) for access to Mount Hyjal. The questgiver is inside the Caverns of Time wandering near the center, and his quest level is 70, so you won't be able to see it if you're too far from 70.
The only place you may earn Scale of the Sands reputation in is within the instance at Mount Hyjal, and only through boss kills.
Since both of these factions are in the same place and don't warrant individual updating, we will cover both here.
Reputation Numbers (Keepers of Time)
Since this is a very straightforward reputation grind, you will not need to kill monsters outside the instances nor worry about many quests other than the ones given to you at the very Caverns of Time. Let's go over your expected Quest Reputation and Monster reputation per kill.
~6000 for Old Hillsbrad Quest Completion
~8000 for Black Morass Quest Completion
Old Hillsbrad: 8 reputation per kill (Trash), 80 reputation per kill (Bosses)
Black Morass: 50 reputation per kill (Rift Lord), 120 reputation per kill (Bosses)
Reputation Numbers (Scale of the Sands)
The Vials of Eternity (Neutral - Friendly, 3000)
Monsters in Mount Hyjal's Instance
Boss Monsters (400 rep per kill)
Archimonde (1000 rep per kill)
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