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 1). 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'Incarnazione, 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'Incarnazione, but year 1 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.