The calendar in use in Pisa from the Middle Ages began on 25 March (the traditional date of the Virgin conception)[a] in the year of Jesus' incarnation (which thus occurred within year one). This method of dating was followed also in Cortona and Pistoia. Dates in the Pisan calendar are said to be in the stile pisano (“Pisan style”) or by the calculus Pisanus (“Pisan calculation”).[b] The Florentine calendar, as well as that of Siena, also belonged to the stile dell'Annunciazione,[c] but year one began one year after Jesus' incarnation.
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.
Not until 1749 were the Pisan and the other ancient calendars definitively outlawed in Tuscany: in that year the recently appointed Grand Duke and Holy Roman Emperor, Francis I, ordered that, starting from 1750, the first of January should become the first day of the year, thus having the "peoples of Tuscia" conform to all the others. A plaque in Latin commemorating the grand ducal decree is affixed to the west wall of the Loggia dei Lanzi, in Florence, Piazza della Signoria.
- The calendar thus belonged to the stile dell'Annunciazione or dell'Incarnazione ("style of the Annunciation" or "of the Incarnation"), usually styled in Latin as ab [Dominica] incarnatione, which began the year with the Feast of the Annunciation (25 March).
- This was in contrast to the Venetian style (more veneto), that began the year on 1 March; the stile della Natività ("style of the Nativity") that began with Christmas (25 December) and was in use in Arezzo, Assisi and Perugia; and the stile della Circoncisione ("Style of the circumcision") that began on 1 January, as in the Gregorian calendar.
- As did the legal year in England, Ireland and the British colonies until 1752 (see Calendar (New Style) Act 1750). In Scotland the legal start of the year had been moved to 1 January in 1600.
- The plaque is reproduced online and translated into Italian at the "FlorencewithGuide" website (Silvia Bonacini, Il calendario fiorentino). Two similar plaques are affixed in Pisa and Siena.
- 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.