That would mean a healing field using 'Time' should just be around each individual requiring aid, or the person can just go back in time and prevent it all together, right?
Is there a limit to how far back or forward they can go?
Well, this is actually a bit complicated to explain. The thing about time magic is that rather than transporting something through time, it controls the time for a process of something. This means that if you're wounded, you can apply a time field on the wounded area to revert the process of the creation of the wound, which would in effect heal it, or you can stop or reverse a bullet in mid-flight, but you can't actually transport something through time.
If, for example, your ally takes a wound in battle, you can use time magic to revert the process of the injury, and thus healing it (this does not mean it will happen as soon as you stop reverting - it's kind of been "cancelled"), but your example of going back in time to prevent the injury from ever happening is impossible due to two things. First, there would be a paradox in which you would no longer have needed to go back in time to prevent the wound since it never happened, but since you never prevented it, it happens, but then you go back in time and prevents it, and then it goes on and on. Second, you would literally break time. This is due to how the time/space continuum is built in this universe. If you want to know more, why not ask the Warden once you meet him.
Also, if you transported a piece of the ground back through time, even by a second, it would be transported to where it would have been in space at that very time (Earth spins, remember), and so it would have to be practically crammed
inside the earth that is already there on an atomic level. This would literally result in a hydrogen bomb, or at least the process that causes the explosion.
Also, advancing or reversing the process of something also ages or rejuvenates it, and this also applies to living beings. One of the offensive capabilities of a time mage is to actually age his enemies to death.