The calendar in use in Pisa from the Middle Ages until at least 1406 (and possibly as late as 1749) began on 25 March (the traditional date of the Virgin conception) 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”). The calendar belonged to the stile dell'Annunciazione ("style of the Annunciation"), which began the year with the Feast of the Annunciation (25 March), in contrast to those of the stile della Natività, which began the year with Christmas (25 December), like the calendars in use in Arezzo, Assisi and Perugia. The Florentine calendar, as well as that of Siena, also belonged to the stile dell'Annunciazione, 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.