Chronomancers are a sub-class of Necron Crypteks, who, as their name implies, like to fuck around with time for work and pleasure. They are armed with Aeonstaves and a bunch of Countertemporal Nanomines. A subsect of Chronomancy is known as Astromancy, whose disciples are able to use the stars to divine the future.
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.
On their off times, they usually try to play techno-fortune telling for their superiors, a bit like the Farseers from the Eldar, but with less probability of getting it wrong and fucking everyone up.
In terms of the delicious crunch, 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. Note that his buff target does not need to be a Core unit. They also happen to move at the same pace as a certain new Destroyer Cult unit...
Also gets a choice of staff, either an 18" anti-tank (with D3+3 damage!) or an assault D3 anti-infantry that ignores invulnerable saves. Extremely good alongside big squads of Warriors or Scarabs, as a 5++ makes those objective campers a nightmare to remove.