Gregorian calendar
No.
Name
Days
1
January
31
2
February
28 or 29
3
March
31
4
April
30
5
May
31
6
June
30
7
July
31
8
August
31
9
September
30
10
October
31
11
November
30
12
December
31
The Gregorian calendar is the internation-
ally accepted civil calendar.[1][2][3] It was
first proposed by the Calabrian doctor Aloysi-
us Lilius, and decreed by Pope Gregory XIII,
after whom it was named, on 24 February
1582 by the papal bull Inter gravissimas.
It
is a reform of the Julian calendar.
Gregory’s bull does not order any particular
year numbering system, but uses the Anno
Domini system which counts years from the
traditional Incarnation of Jesus, and which
spread throughout Europe during the middle
ages. That is the same year numbering sys-
tem that is the de facto international stand-
ard today.[4]
The Gregorian calendar modifies the Juli-
an calendar’s regular four-year cycle of leap
years as follows:
Every year that is exactly divisible
by four is a leap year, except for
years that are exactly divisible by
100; the centurial years that are ex-
actly divisible by 400 are still leap
years. For example, the year 1900 is
not a leap year; the year 2000 is a
leap year.[5]
Description
The Gregorian solar calendar is an arithmet-
ical calendar. It counts days as the basic unit
of time, grouping them into years of 365 or
366 days; and repeats completely every
146,097 days, which fill 400 years, and which
also happens to be 20,871 seven-day weeks.
Of these 400 years, 303 (the "common
years") have 365 days, and 97 (the leap
years) have 366 days. This gives an average
year length of exactly 365.2425 days, or 365
days, 5 hours, 49 minutes and 12 seconds.
A Gregorian year is divided into twelve
months of irregular length, that is, with no
regular relationship among their lengths:
Detail of the tomb of Pope Gregory XIII cel-
ebrating the introduction of the Gregorian
calendar.
A calendar date is fully specified by the year
(numbered by some scheme beyond the
scope of the calendar itself), the month (iden-
tified by name or numb