Q. What is the name of the modern calendar?

Answer

The Gregorian calendar based on the motion of the Earth around the Sun is the most used system of dates worldwide. Named after Pope Gregory XIII who in 1582 published the papal bull Inter gravissimas announcing calendar modifications for all of Catholic Christendom. The Gregorian calendar established by Pope Gregory XIII in 1582 consists of 365 days plus an additional day every four years (the leap year) except for years that are divisible by 100 but not by 400.