The calendar

Calendar

A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...

in use in Pisa

Pisa

Pisa is a city in Tuscany, Central Italy, on the right bank of the mouth of the River Arno on the Tyrrhenian Sea. It is the capital city of the Province of Pisa...

, the stile pisano (“Pisan style”) or calculus Pisanus (“Pisan calculation”), from the High Middle Ages

High Middle Ages

The High Middle Ages was the period of European history around the 11th, 12th, and 13th centuries . The High Middle Ages were preceded by the Early Middle Ages and followed by the Late Middle Ages, which by convention end around 1500....

 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

Incarnation (Christianity)

The Incarnation in traditional Christianity is the belief that Jesus Christ the second person of the Trinity, also known as God the Son or the Logos , "became flesh" by being conceived in the womb of a woman, the Virgin Mary, also known as the Theotokos .The Incarnation is a fundamental theological...

 (which thus occurred within year 1). This method of dating was followed also in Cortona

Cortona

Cortona is a town and comune in the province of Arezzo, in Tuscany, Italy. It is the main cultural and artistic center of the Val di Chiana after Arezzo.-History:...

 and Pistoia

Pistoia

Pistoia is a city and comune in the Tuscany region of Italy, the capital of a province of the same name, located about 30 km west and north of Florence and is crossed by the Ombrone Pistoiese, a tributary of the River Arno.-History:...

. 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

Florence

Florence is the capital city of the Italian region of Tuscany and of the province of Florence. It is the most populous city in Tuscany, with approximately 370,000 inhabitants, expanding to over 1.5 million in the metropolitan area....

 and Siena

Siena

Siena is a city in Tuscany, Italy. It is the capital of the province of Siena.The historic centre of Siena has been declared by UNESCO a World Heritage Site. It is one of the nation's most visited tourist attractions, with over 163,000 international arrivals in 2008...

. 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

Gregorian calendar

The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

. 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.