Gregorian calendar, system of measuring years used by most of the world today. Developed by Pope Gregory XIII in 1582, it replaced the Julian calendar established by Julius Caesar in 46 B.C. The Julian calendar year, which was about 11 minutes longer than the solar year, had become 10 days ahead by A.D. 1580. Gregory dropped 10 days from October 1582 to bring the calendar year into conformity with the solar year. He also established leap year.