Origin of calendar dating
In Mesopotamia the solar year was divided into two seasons, the "summer," which included the barley harvests in the second half of May or in the beginning of June, and the "winter," which roughly corresponded to today's fall-winter.
Three seasons (Assyria) and four seasons (Anatolia) were counted in northerly countries, but in Mesopotamia the bipartition of the year seemed natural. 1800 BC the prognoses for the welfare of the city of Mari, on the middle Euphrates, were taken for six months.
All the dates will be the same for dates before September 1582, while the Julian calendar was in general use, and they are again all in agreement after 1918 by which time the Gregorian calendar had been universally adopted.By 1000 AD, European countries had generally adopted the Julian calendar of 365 days in a year and 366 days every fourth year.The Julian calendar did however accumulate errors at a rate of roughly one day every hundred years when compared to astronomical events.In a sixth-century treatise on the calculation of Easter, Dionysius 'the Little' first proposed to count from the birth of Christ to avoid honouring the hated persecutor Diocletian.His idea was popularised in England by the Venerable Bede, who added the notion of counting backwards for dates 'Before Christ'.