<P> The calendar era before and under the Roman kings is uncertain but dating by regnal years was common in antiquity . Under the Roman Republic, from 509 BC, years were most commonly described in terms of their reigning ordinary consuls . (Temporary and honorary consuls were sometimes elected or appointed but were not used in dating .) Consular lists were displayed on the public calendars . After the institution of the Roman Empire, regnal dates based on the emperors' terms in office became more common . Some historians of the later republic and early imperial eras dated from the legendary founding of the city of Rome (ab urbe condita or AVC). Varro's date for this was 753 BC but other writers used different dates, varying by several decades . Such dating was, however, never widespread . After the consuls waned in importance, most Roman dating was regnal or followed Diocletian's 15 - year Indiction tax cycle . These cycles were not distinguished, however, so that "year 2 of the indiction" may refer to any of 298, 313, 328, &c . The Orthodox subjects of the Byzantine Empire used various Christian eras, including those based on Diocletian's persecutions, Christ's incarnation, and the supposed age of the world . </P> <P> The Romans did not have records of their early calendars but, like modern historians, assumed the year originally began in March on the basis of the names of the months following June . The consul M. Fulvius Nobilior (r . 189 BC) wrote a commentary on the calendar at the Temple of Hercules Musarum that claimed January had been named for Janus because the god faced both ways, suggesting it had been instituted as a first month . It was, however, usually said to have been instituted along with February, whose nature and festivals suggest it had originally been considered the last month of the year . The consuls' term of office--and thus the order of the years under the republic--seems to have changed several times . Their inaugurations were finally moved to 1 January (Kal . Ian .) in 153 BC to allow Q. Fulvius Nobilior to attack Segeda in Spain during the Celtiberian Wars, before which they had occurred on 15 March (Eid . Mart .). There is reason to believe the inauguration date had been 1 May during the 3rd century BC until 222 BC and Livy mentions earlier inaugurations on 15 May (Eid . Mai .), 1 July (Kal . Qui .), 1 August (Kal . Sex .), 1 October (Kal. Oct .), and 15 December (Eid . Dec .). Under the Julian calendar, the year began on 1 January but years of the Indiction cycle began on 1 September . </P> <P> In addition Egypt's separate calendar, some provinces maintained their records using a local era . Africa dated its records sequentially from 39 BC; Spain from AD 38 . This dating system continued as the Spanish era used in medieval Spain . </P> <P> The continuity of names from the Roman to the Gregorian calendar can lead to the mistaken belief that Roman dates correspond to Julian or Gregorian ones . In fact, the essentially complete list of Roman consuls allows general certainty of years back to the establishment of the republic but the uncertainty as to the end of lunar dating and the irregularity of Roman intercalation means that dates which can be independently verified are invariably weeks to months outside of their "proper" place . Two astronomical events dated by Livy show the calendar 4 months out of alignment with the Julian date in 190 BC and 2 months out of alignment in 168 BC . Thus, "the year of the consulship of Publius Cornelius Sciopio Africanus and Publius Licinius Crassus" (usually given as "205 BC") actually began on 15 March 205 BC and ended on 14 March 204 BC according to the Roman calendar but may have begun as early as November or December 206 BC owing to its misalignment . Even following the establishment of the Julian calendar, the leap years were not applied correctly by the Roman priests, meaning dates are a few days out of their "proper" place until a few decades into Augustus's reign . </P>

Who added january and february to the calendar