Real Dictionary

Gregorian calendar in American English
a corrected form of the Julian calendar, introduced by Pope Gregory XIII in 1582 and now used in most countries of the world: it provides for an ordinary year of 365 days and a leap year of 366 days every fourth, even year, exclusive of the final year of
noun: the reformed Julian calendar now in use, according to which the ordinary year consists of 365 days, and a leap year of 366 days occurs in every year whose number is exactly divisible by 4 except centenary years whose numbers are not exactly divisible by 4

Gregorian calendar in British English
noun: the revision of the Julian calendar introduced in 1582 by Pope Gregory XIII and still in force, whereby the ordinary year is made to consist of 365 days and a leap year occurs in every year whose number is divisible by four, except those centenary years,