Pisan calendar

The calendar in use in Pisa, the stile pisano (“Pisan style”) or calculus Pisanus (“Pisan calculation”), from the High Middle Ages until at least 1406 (and possibly as late as 1749)^[1] began on 25 March (the traditional date of the Virgin conception) in the year of Jesus' incarnation (which thus occurred within year 1). This method of dating was followed also in Cortona and Pistoia. A similar calendar (both belonged to the stile dell'Incarnazione, which began the year with the Feast of the Annunciation) which likewise began the year on 25 March calculated the years from Jesus' incarnation (which thus occurred in the calendar year before year 1) was in use in Florence and Siena.^[2] In general, it is necessary to subtract one year from a date given in the Pisan calendar to obtain one given in the standard Gregorian calendar. This does not apply to dates between 1 January and 24 March inclusive, which are given in the same year in both calendars but consist in the final days of the Pisan calendar and the first of the Gregorian.

References

Cohn, Samuel Kline. The Cult of Remembrance and the Black Death: Six Renaissance Cities in Central Italy. Baltimore: Johns Hopkins University Press, 1997, p. xiii.

Notes

^ Adriano Capelli cites 1749 as the date when the practice changed, but an examination of documents indicates that the change had mostly occurred in 1406, when Florence conquered Pisa.
^ This contrasts with the stile della Natività, which began the year with Christmas (25 December), like the calendars in use in Arezzo, Assisi and Perugia, which were otherwise the same as the Florentine.