WordReference Random House Unabridged Dictionary of American English © 2020
Gregorian calendar, 
  • Timethe 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 400, as 1700, 1800, and 1900. See table under  calendar. 
    • 1640–50; named after Pope Gregory XIII; see -ian

    Collins Concise English Dictionary © HarperCollins Publishers::
    Gregorian calendar n
    1. 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, such as 1900, whose numbers are not divisible by 400
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