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.
Indian national calendar Usage started officially at 1 Chaitra 1879, Saka Era, or 22 March 1957. However, government officials seem to largely ignore the New Year's Day of this calendar in favour of the religious calendar.[3]