46 BC

Year 46 BC was the last year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Caesar and Lepidus (or, less frequently, year 708 Ab urbe condita). The denomination 46 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

This year marks the change from the pre-Julian Roman calendar to the Julian calendar. The Romans had to periodically add a leap month every few years to keep the calendar year in sync with the solar year but had missed a few with the chaos of the civil wars of the late republic. Julius Caesar added Mercedonius (23 days) and two other intercalary months (33 and 34 days respectively) to the 355-day lunar year, to recalibrate the calendar in preparation for his calendar reform, which went into effect in 45 BC. The resulting calendar year, the longest calendar year in recorded history, lasted 445 days – nearly 80 days longer than the sidereal year (the orbit of Earth around the Sun) – and was nicknamed the annus confusionis ("Year of Confusion").