The Julian calendar began in 45 BCE, following a reform ordered by Julius Caesar in 46 BCE. The distinction between the reform year and the first full year of the new system explains why sources sometimes mention both dates.
The short answer: 45 BCE
Julius Caesar reorganized the Roman calendar after years of irregular intercalation had left the civic calendar poorly aligned with the seasons. The reform was prepared during 46 BCE, and the new calendar took effect at the beginning of 45 BCE.
Modern reference works often describe 46 BCE as the year of reform and 45 BCE as the first year of the Julian calendar. That is not a contradiction: one date refers to the transition, the other to the operation of the new system.
Why 46 BCE was unusually long
Before the reform could begin cleanly, the Roman calendar had to be brought back into seasonal alignment. Additional days were inserted into 46 BCE, making the transition year exceptionally long. Ancient chronologies differ in the details of Roman intercalation, but the broad purpose is clear: reset the calendar before adopting a more regular solar scheme.
What changed under the Julian system
- A common year contained 365 days.
- An additional day was inserted on a four-year cycle, producing an average calendar year of 365.25 days.
- The reform reduced dependence on irregular political decisions about intercalary months.
- The month structure developed into the framework inherited by later European calendars, although some details of leap-day administration were corrected after Caesar’s death.
Why the calendar eventually drifted
A tropical year is slightly shorter than 365.25 days. The difference is only minutes per year, but over many centuries it accumulated. By the sixteenth century the seasonal reference used for the calculation of Easter had shifted enough to motivate the Gregorian reform of 1582.
The Gregorian calendar kept the familiar month structure while changing the leap-year rule. Century years are not leap years unless divisible by 400, giving an average year of 365.2425 days.
Julian calendar today
The Julian calendar is no longer the dominant civil calendar, but it remains culturally and religiously important. Some Eastern Orthodox and Oriental Christian communities continue to use it for parts of their liturgical calendar, while historians also use Julian dates when working with documents produced before local adoption of the Gregorian calendar.
