The
Gregorian calendar, also known as the
Western calendar, or
Christian calendar, is the internationally accepted
civil calendar.
It was introduced by
Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a
papal bull known by its opening words
Inter gravissimas.
The reformed calendar was adopted later that year by a handful of countries, with other countries adopting it over the following centuries. The motivation for the Gregorian reform was that the
Julian calendar assumes that the time between
vernal equinoxes is 365.25 days, when in fact it is presently almost exactly 11 minutes shorter. The error between these values accumulated at the rate of about three days every four centuries, resulting in the equinox occurring on March 11 and moving steadily earlier in the Julian calendar at the time of the Gregorian reform. Since the Spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered that this steady movement in the date of the equinox was undesirable.