<human>:  When did the Romans start using the Julian calendar?
<bot>: The Julian calendar was introduced in 45 BC by Julius Caesar. It was adopted throughout the Roman Empire, but it was not universally accepted until the 6th century. The Julian calendar was a leap year system that was 365 days long. It was not a perfect system, as it had many quirks, such as the fact that the Julian calendar was 13 days behind the Gregorian calendar.