In Superstring Theory there are 11 dimensions, 10 spatial, 1 temporal. 7 of those 10 spatial dimensions are circular (or the rough 7 dimensional equivalent, Joyce Manifolds). This means that after travelling along one of those dimensions for a while you will end up at the same point again. A curled up temporal dimension will be similar, in that traversing it will result in returning to the same point again.

Just as we do not know whether the normal three extended spatial dimensions are circular or not, we do not know whether time is similarly curled. However, beyond the normal problems of trying to measure, say, the radius of the dimension, in time we have the added complication of only being able to traverse it in one direction. In fact, this means that we pretty much cannot distinguish between a curled up temporal dimension and a "straight" one.