Leap year inserted on the first or second day after the Terminalia (a. d. VII Kal. Mar., February 23). The remaining days of Februarius were dropped. This intercalary month, named Intercalaris or Mercedonius, contained 27 days. The religious festivals that were normally celebrated in the last five days of February were moved to the last five days of Intercalaris. Because only 22 or 23 days were effectively added, not a full lunation, the calends and ides of the Roman Republican calendar were no longer associated with the new moon and full moon. The Julian calendar, which was developed in 46 BC by
Unit of time year cannot be divided into 12 28-day months since 12 times 28 is 336, well short of 365. The lunar month (as defined by the moon's rotation) is not 28 days but 28.3 days. The year, defined in the Gregorian calendar as 365.24 days has to be adjusted with leap days and leap seconds. Consequently, these units are now all defined as multiples of seconds. Units of time based on orders of magnitude of the second include the nanosecond and the millisecond. The natural units for timekeeping used by most historical societies are the day, the solar year and the