who came up with the modern day calendar
History of calendars The Gregorian calendar was introduced as a refinement of the Julian calendar in 1582, and is today in worldwide use as the de facto calendar for secular purposes.
Civil calendar The most widespread civil calendar and de facto international standard is the Gregorian calendar. Though that calendar is associated with the Catholic Church and the papacy, it has been adopted, as a matter of convenience, by many secular and non-Christian countries although some countries use other calendars.