Officially called the Harbingers of Eternity, Chronomancers are pretty much exactly what you'd expect. Guys who mess around with the flow of time in the battlefield to give their metallic buddies an advantage over their fleshy opponents. Doing stuff like slowing down incoming fire from the enemy so their own troops can just stroll past without a scratch.
In terms of what we know of the actual crunch at the moment, the Chronometron piece of equipment has been made their main gimmick ability. With it allowing you to select a friendly unit during your command phase to reroll charge rolls and grant it a 5+ invul phase.