<human>:  When did the Romans start using the Julian calendar?
<bot>: The Julian calendar was introduced in 45 BC by Julius Caesar. The Julian calendar is a solar calendar, meaning that it is based on the cycle of the Earth's revolution around the Sun. The Julian calendar is a leap year system, meaning that every four years a year is added to the calendar. This means that the year 1 AD is a leap year, but the year 2 AD is not a leap year.