The months of January and February did not feature in earlier versions of the Roman calendar, which divided the year into 10 months and left 61 days unaccounted for in the winter. They were added around 700 BCE.

January and February were initially the last months of the calendar year, which began in March (Martius). However, in 450 BCE, the beginning of the year was moved to January 1.

January originally consisted of 30 days when it was added to the 10-month Roman calendar. However, a day was added making it 31 days long in 46 BCE by Julius Caesar.