It's what it sounds like, a caster that works primarily through the manipulation of time.
Spells would be mostly arcane or shadow tree, but talents would be Past, Present, and Future.
Past would focus mostly on altering the past or moving enemies back to the past. Moving enemies back would result in stat losses since they are going back to a lesser-geared, less powerful existence. This wouldn't actually reverse gearing, just a % change to stats. The ultimate Past ability would be Save-Undo, allowing the player to save a situation and then revert all allies back to that state if desired. A common PvE use would be rather boring, saving your place before a phase change in order to not have to redo it after each learning wipe in the next.
Future would mostly be self-buffs or damage boosts, moving ahead in time for either predictive value or to hasten a desired outcome. A simple spell would be a future bubble, moving the caster into a later time and allowing them to avoid any attacks. They could also attack future enemies, allowing for an interesting source of burst damage by attacking in the future, then unloading a second burst when that time is reached in the present. The ultimate ability would be Fast Forward. It would take the average events of the past 10 seconds and then move ahead 60 seconds, acting as if the past 10 seconds were repeated over that 60. This would be a huge raid DPS boost, allowing players to pop all trinkets and cooldowns for the 10 seconds in order to get a vastly magnified effect. There is great risk though. If the tank does not get enough healing during the ten seconds, he may die during the minute forward. DPS may pull aggro at the end. Mana, rage, and energy would not be changed except maybe in the positive due to regen over the ten seconds.
Present would manipulate the current time and unfortunately I cannot think of anything cool to do right now. Haste is the future, slow is the past. The present seems more like the realm of everyone else. Maybe there could be an alternate timeline, allowing one to duplicate oneself or allies.