Pisan calendar
The Pisan calendar, also referred to as the stile pisano or the calculus Pisanus, was the calendar used in the Republic of Pisa in Italy during the Middle Ages, which differed from the traditional Julian calendar.
Beginning of the year
The Pisan year began on 25 March, and not on 1 January, with the apparent year lying ahead of the traditional Julian calendar. Thus, 24 March 1200 was followed by 25 March 1201, and 31 December 1201 would then be followed by 1 January 1201, which was the point at which the two calendars synchronised. This is the reason that most dates have an apparent discrepancy of one year, as the two calendars differ for just over nine months of each cycle. For example, a birth date of 10 September 1552 in Pisan reckoning translates to 10 September 1551 in the Julian calendar.Beginning the year on a date other than 1 January was common during the mediaeval period. The first day of the year falling on 25 March meant that the Pisan calendar was in the stile dell'Annunciazione or stile dell'Incarnazione - also styled in Latin as ab incarnatione - by reference to the Solemnity of the Annunciation, and similar calendars saw use in Cortona and Pistoia. The Florentine, Sienese, English and Scottish calendars were also in this style, but confusingly ran behind the Julian calendar rather than ahead, resulting in them lying exactly one year behind of the Pisan calendar. By contrast, calendars in the stile della Natività as in Arezzo, Assisi and Perugia began on the Solemnity of the Nativity of the Lord on 25 December, the Venetian calendar began on 1 March until the Fall of the Venetian Republic, and the French year on Easter day until 1564. The traditional Julian calendar was sometimes said to be in the stile della Circoncisione. See beginning of the year.