Even with current knowledge, we're not really omniscient about what happened in the past. I know the Romans were good record-keepers (I'm not sure how good), but I would be surprised if there weren't at least some errors and omissions in the small details.

If we did have omniscient knowledge, I would suggest that the situation might be most analogous after several years - when you could still make predictions with high accuracy, but never really being sure that something you've done has slightly changed the dates or other details of specific events.