The Anno Domini system for numbering years was invented in 525 by Dionysius Exiguus. Apparently Bede was instrumental in popularising it. Before that, I think it was generally since the founding of Rome (AUC), which is assumed to have been 753BC, though the Romans didn't bother with this, using Emperors' regnal years. So it's quite sensible to assume that in some sci-fi future, especially if post-apocalyptic, we'll have abandoned this system and started again from scratch.