It's a good thing the journeys kept to a fairly narrow range - maybe the machine had limits on how far back or forwards it could travel. We only have four digits for the year, so no visiting the year 10,000 nor can we select a date BC (or BCE). Unless maybe it uses a digit for a - sign, that would stretch it all the way to 999 BC. So no going to take a look at the pyramids being built. Anyhow, that raises questions of who's years do you use, do you roll the current calendar back, which would leave you some days out from the calendar of the era? Never mind the Romans adding and removing months, so sticking to modern calendar seems best, but you'd need to have a conversion chart if you wanted to hit a specific day.