- Gregorian Calendar
 - Summary of the Gregorian calendar, the calendar currently in use in most of the Western world, the mathematical rules it uses, and the history of its slow adoption in the 17th and 18th centuries. - Gregorian Calendar Definitions
- Answers common questions about the Gregorian calendar, the most common calendar in use today, including what years are leap years, when the third millenium really started, and what the 4000-year rule is.
- Inter Gravissimas
- Text of the papal bull proclaiming the Catholic Church's adoption of the Gregorian calendar, signed by Pope Gregory XIII in 1582. In English, French, and Latin.
- Perpetual Calendar, The
- Article about the process of switching from the Julian to the Gregorian calendar, with a table showing when each nation made the transition.
- Synoptical Julian-Gregorian Calendar
- Compare the Julian and Gregorian calendars for any date between 1582 and 2100 using this side-by-side reference.
- Wikipedia: Gregorian Calendar
- Encyclopedia entry about the Gregorian or New Style calendar, the twelve-month calendar introduced in the Western world in 1582 to replace the less precise Julian calendar.
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