Year 300 (CCC) was a leap year starting on Monday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Constantius and Valerius (or, less frequently, year 1053 Ab urbe condita). The denomination 300 for this year has been used since the early Middle Ages / Medieval period, when the Latin language term / abbreviation "Anno Domini" ("In the year of Our Lord") for the calendar era became the prevalent universal / worldwide method for naming and numbering years. First beginning in Europe at the end of the Roman Empire (after the split of the Western Roman Empire and Eastern Roman Empire (later Byzantine Empire) in the early Middle Ages / Medieval period.
Then the Christian-oriented dating system then spreading west across the Atlantic Ocean with the Western European explorers and religious faith to the continents of the Americas of the Western Hemisphere, then through the simultaneous movement of the various Christian churches, and Europeans along sea trading routes with the military / political / economic / social influences of Colonialism / Imperialism spread worldwide to Africa, Asia and Australia / Oceania.