Merriam-Webster Collegiate Dictionary - gregorian calendar

 noun  Date: circa 1771 a calendar in general use introduced in 1582 by Pope Gregory XIII as a revision of the Julian calendar, adopted in Great Britain and the American colonies in 1752, marked by the suppression of 10 days or after 1700 11 days, and having leap years in every year divisible by four with the restriction that centesimal years are leap years only when divisible by 400 — see month table