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Inter gravissimas is a

issued by

on

,

. It is the founding document for the Gregorian calendar, which is used in most countries.

This bull declares that:

  • , will be followed by , (paragraph 7);
  • these 10 missing days will not be counted in calculating end days of loans, etc. (paragraph 8);
  • centennial years, such as , , and will not be leap years, but years that can be divided by 400, such as and will be (paragraph 9);
  • Easter will be computed via new Paschal tables (paragraph 10).

The name of the bull consists of the first two words of the bull, which starts: "Inter gravissimas pastoralis officii nostri curas " ("Among our most serious pastoral duties ").

Attached to the bull were six canons which enabled the computation of the new Gregorian Easter and two calendars listing

s' days, one for the last 2½ months of 1582 and another for the entire new Gregorian year. The bull, canons, and calendars were reprinted as part of the front matter of the principal book explaining and defending the Gregorian calendar,

, Romani calendarij à Gregorio XIII. P. M. restituti explicatio (1603), which is tome V in his collected works Opera Mathematica (1612), available in the microfiche collection Landmarks II monographs.

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