The expression Anno Domini (abbreviated as AD) means "the year of the Lord" and is used to denote the number of years from the one in which they would place the Incarnation and Nativity of Jesus Christ.

   Anno Domini is an abbreviation of the Latin formula is "annus ab Incarnatione Domini nostri Jesu Christi" (the year from the Incarnation of our Lord Jesus Christ), and the formula Latin "annus a Nativitate Domini nostri Jesu Christi" (year of the Nativity of our Lord Jesus Christ).

   This system of numbering the years according to the Anno Domini is currently dominant in many countries of the world, both for commercial use and for scientific ones. For several decades, in fact, it is the standard recognized by international institutions such as the United Nations and the Universal Postal Union, which suggests to its offices to accompany such date to any other date chosen in the extension of its documents.

   The years after the birth of Jesus "post Christum natum" are also indicated with synonyms:
A.D. "Anno Domini";
d.C. "after Christ";
e.V. "Common Era";
C.E. "Common Era" in English-speaking countries;
u.Z. "unserer Zeitrechnung" in the former GDR.

   Previous ones instead with synonyms:
a.C. "before Christ";
a.e.V. "before Common Era";
B.C. "Before Christ" or B.C.E. "Before Common Era" in English-speaking countries;
v.u.Z. "vor unserer Zeitrechnung" in the former GDR.

   Although the term Anno Domini was in widespread use since the ninth century, the term "Before Christ" and synonyms was not widespread until the late fifteenth century.

Julian calendar

   The Julian calendar is a solar calendar, that is based on the cycle of the seasons. It was developed by the astronomer greek Sosigenes of Alexandria and promulgated by Julius Caesar (hence the name), in his capacity as Pontifex Maximus, in the year 46 BC .

   The Julian reform, in essence, took up the Egyptian calendar reformed by Decree of Canopus and stared at the start of the year on 1 January, while the first was on 1 March.

   The names of the months of the Julian calendar are those derived from the ancient Roman calendar, with some changes introduced by the emperors:

1. Ianuarius: month dedicated to Janus (Janus), two-faced god, symbolically marking the transition from the previous year to the next. Ianua in Latin means "door", another reference to the change of the year.
2. Februarius: sabina Februa comes from the word meaning "purification" in this month was practiced purification of the fields before they were cultivated.
3. Martius: month dedicated to Mars, the god of war.
4. Aprilis: is thought to derive from the Etruscan word Apru and refers to Aphrodite, that is Venus.
5. Maius: dedicated to Maia, the goddess of fertility, this month was practiced in a ritual designed to assure the fertility of the fields.
6. Iunius: dedicated to the goddess Juno, that is Juno.
7. Iulius: Julius Caesar dedicated to Gaius Julius Caesar.
8. Augustus: Gaius Julius Caesar Octavianus dedicated to Augustus, the emperor Octavian Augustus.
9. September: the seventh month of the ancient calendar of Romulus, who saw in March as the first month.
10. October: the eighth month of the calendar of Romulus.
11. November: the ninth month of the calendar of Romulus.
12. December: the tenth month of the calendar of Romulus.

   The seventh month (Quintilis) was dedicated to Julius Caesar in 44 BC by Marco Antonio, the eighth (sextilis) to Octavian Augustus in the year 8 BC (Lex de Pacuvia canteens august). Some texts date the name change in August year 26 or year 23 BC but the date of Lex Pacuvia is certain.

   Although the week solely made up of seven days (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday), already lay in the Egyptian calendar, in the year 321 AD Emperor Constantine decreed permanently. In addition, it was decreed on Sunday (dies solis) day of rest dedicated to the Sun God, instead of Saturday, traditional not only for Jews but for the Gentiles as if Jesus Christ had died on the sixth day of the Jewish week, should be resurrected Sunday.

   On the other hand, it also met another very popular religion: the cult of Mithras, god of covenants and friendship in the Persian religion of the Vedic period who worshiped the sun and its derivative Sol Invictus (Invincible Sun), hence the association between the sun and Jesus Christ by Constantine worked to promote this new, but unknown to most, religion (Christianity).

   The Julian calendar was by then the official calendar of Rome and its domains, then its use spread to all the countries of Europe and America, as they were Christianized. Year 1582 A.D. has been replaced by the Gregorian calendar by decree of Pope Gregory XIII; different countries but have continued to use the Julian calendar, then adapting at different times between the eighteenth and twentieth centuries. Some churches are part of the Orthodox Church still uses the Julian calendar as its liturgical calendar. The Julian calendar is also the basis of the traditional calendar Berber North Africa.

   In the Julian calendar, the bissextile years are used to compensate for the fact that the length of the tropical year (or solar year) is not given by an integer number of days. The extra day is added after February 24. It should be remembered that the Romans were in the habit of counting the days to monthly subtracting certain festivals counting the day of departure, so between February 24 and March 1, there are exactly six days (24-25-26-27-28-1).

   In bissextile years, with 29 days of February, the 24th, which was "Sexto die" would become "Septimo die". But since "Septimo die" was 23 days, not being able to call the 24 "Septimo die" called him "bis Sexto die". Hence the name "bissextile year".

   Sosigenes has established that one every four years is a bissextile year: in this way the average Julian year was of 365 days and a quarter. As a result, the Julian calendar is cyclical every 4 years equivalent to 365 × 4 +1 = 1461 days (not divisible by 7) and, considering the days of the week, the Julian calendar is cyclical every 1461 × 7 = 10227 days, equivalent to a 4 × 7 = 28 years. The difference with the tropical year is thus only 11 minutes and 14 seconds, a precision very accurate for its time.

   This difference of about one hundredth day, but accumulated over the centuries, so the start date of the seasons moved (you lose one day every 128 years). This phenomenon was well known to astronomers medieval; Dante mentions in the Divine Comedy: « Ma prima che gennaio tutto si sverni per la centesma ch'è là giù negletta » (Paradiso XXVII, 142-143).

   For this reason, in the year 1582 AD was introduced to the Gregorian calendar, which reduces the error in just 26 seconds (one day every 3323 years approximately).

   After the birth of Christ are bissextile years the years whose number is divisible by 4. Before the birth of Christ, however, there was no fixed rule, since the application of the rule was assigned to discretionary policy decisions: it was only Octavian Augustus (8 BC) to impose the final determination of the Cesarean bissextile annuality.

   The first bissextile year was 45 BC, the year in which the new calendar came into effect. This is sometimes called the year of confusion, as they had to add 85 days to compensate for the accumulation of errors in the past and restore the vernal equinox to 25 March. In order were added two months between November and December, one of 33 days and the other of 34 days.

   This confusion had various repercussions over the subsequent 50 years until to around 8 AD. In fact, after the death of Julius Caesar (44 BC) were committed various errors making bissextile years many not bissextile years. It was then Augustus in 8 BC that,  to fix the error, ordered that a number of bissextile years should not have been considered bissextile years.

   There is no unanimity of opinion about what years were actually bissextile before reorganization Augustan, simple hypothesis is that they were 45 BC, 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC , 21 BC, 18 BC, 15 BC, 12 BC, 9 BC and 8 AD (skipping leap years 5 BC, 1 BC and 4 AD). It would have been misunderstood that indication to include a bissextile year after three normal years, inserting instead a bissextile year every three years, including bissextile year (ie one every three instead of one every four).

Dionysius Exiguus (Founder of the historical chronology general)

   As of the end of the Republican period (year 46 BC), the Julian calendar enumerates the years since the founding of the city of Rome (Ab Urbe Topped) until the Christian period after the Emperor Constantine.

   In the year 525 AD, Dionysius the Small (Latin: Dionysius Exiguus), a learned monaco native of Scythia, who lived in Rome between the late fifth and early sixth century, developed the calculations chronograph at the base of the system "Anno Domini" (the so-called Christian era).

   The papal chancellor gave him the task to develop a mathematical method to predict the date of Easter according to the rule adopted by the Council of Nicaea (also called "Alexandrian rule").

  Dionysius the Small understood that the dates of Easter are repeated cyclically every 532 years in the Julian calendar (the calendar which was in force at the time) and compiled a table (below in excerpt), which contained a list of dates throughout the duration of this cycle:

  A       B      C       D    E          F                  G            H
532   10   nulla    4   17   5 Aprilis       11 Aprilis     20
533   11     11      5   18   25 Martius   27 Martius    16
534   12     22      6   19   13 Aprilis     16 Aprilis     17
… … … … … … … …

A = calendar year;
B = number of indictio;
C = epact;
D = competitor;
E = number of cycle of the moon;
F = date of Alexandrian Paschal full moon;
G = date of Alexandrian Easter Sunday;
H = age of the moon at the date of Alexandrian Paschal Sunday.

   The table of Dionysius was officially adopted and was used by the Catholic Church until the Gregorian reform, which took place in the year 1582, while the Orthodox Church, which has not signed the reform, the still use.

   In compiling his table of dates of Easter, Dionysius chose to number the years according to a criterion which is entirely new, compared to those that were used at that time (from the founding of Rome or from the beginning of the reign of Diocletian or from the beginning of time, the latter calculated according to the conventional age of the Biblical patriarchs) that was to count "ab Incarnatione Domini nostri Jesu Christi", meaning "from the Incarnation of our Lord Jesus Christ."

   Dionysius calculated that the year of the Incarnation (March 25) of Jesus Christ, the same as the birth (25 December), coincided with the 753 th year after the year of the founding of Rome (Ab Urbe Topped).

   Dionysius did not address the issue of the number of years before the birth of Jesus Christ, in fact, was only decided at a later date to call "year 1 BC" the 753 th year after the year of the foundation of Rome.
   In fact, Dionysius called the '"Year 1 AD", which follows "year 1 BC", the first year of the Christian era, there is no "year 0 AD", Dionysius did not know the "0", in fact, the Latin word "nulla" in the third column of his Easter table does not mean "0".

   In medieval Europe, the "0" was not introduced until the second millennium of the Christian era.

   So, being the founder of the Christian era, nowadays together with the Gregorian calendar (since 1582) by far the most extensive chronological system on earth, Dionysius was also the founder of historical chronology general.

   Dionysius calculated the canonical date of the birth of Jesus Christ using the Gospels and the historical documents available to him, which did not indicate precisely the date of birth of Jesus Christ.

   The traditional date for the birth of Jesus Christ is therefore 25 December of the year 1 BC, a week before the beginning of the year 1 AD.

   Properly, according to Christian doctrine, the moment of the Incarnation of Jesus Christ is that of his conception, not of his birth, but if Jesus Christ is born December 25, according to tradition, conception and birth occurred in the same year (the conception we celebrate the feast of the Annunciation on March 25, exactly nine months before Christmas).

This date is now the subject of debate: some authors claim that Dionysius has incorrectly calculated the date of birth of Jesus Christ that, according to the interpretation of the Gospels, should be placed between year 7 and year 4 BC.

   The commonly accepted date for the death of Herod the Great, in whose reign Jesus Christ was born, is in fact the 4 BC.

   Jesus Christ, therefore, can not be born after that date, this hypothesis does not respect the tradition which calculates to 33 years, the age of Jesus Christ at the time of his crucifixion.

   Given that the Gospel of Luke states that the preaching of John the Baptist began "in the fifteenth year of the reign of Tiberius Caesar", in the year 29 AD, and that the subsequent preaching of Jesus Christ lasted at least three years, anticipate the birth of Jesus Christ from 7 to 4 years compared to calculations of Dionysius is to say that Jesus Christ died at the age between 29+3+7=39 years and 29+3+4=36 years.

   On the contrary, if historians confirm the hypothesis that Herod is dead in 3 AD, after the birth of Jesus Christ, while in 4 BC has associated with himself his children in the kingdom, the Dionysius's calculation would be correct.

   The numbering of Dionysius spread throughout the Christian world, initially in Italy, in the tables of Easter cycles and in the chronicles. Around the seventh century also in the public and private documents. In the eighth century spread in the acts of sovereign , Francs and British, , while in the tenth century is known throughout Western Europe.

Gregorian calendar

   The Gregorian calendar is the official calendar of most countries of the world. It is named after Pope Gregory XIII, who introduced it in the year 1582, with the papal bull "Inter gravissimas" promulgated from his residence in Villa Mondragone (at Monte Porzio Catone, Rome). It is a modification of the Julian calendar, which was previously in force, in accordance with the proposal made by Luigi Lilio.

   It is a solar calendar that is based on the cycle of the seasons. The year consists of 12 months of different lengths (28 to 31 days), for a total of 365 or 366 days. The years of 366 days are called bissextile years.

   The names of the months of the Gregorian calendar are:
1. January (31 days)
2. February (28 days, 29 in bissextile years)
3. March (31 days)
4. April (30 days)
5. May (31 days)
6. June (30 days)
7. July (31 days)
8. August (31 days)
9. September (30 days)
10. October (31 days)
11. November (30 days)
12. December (31 days).

   The days of each month are identified by consecutively numbers, starting with 1. Thus, for example, the first day of the year is January 1, and the last is December 31.

   As mentioned above, according to the Julian calendar, one every four years is a bissextile year (the number is a multiple of 4).

   The average year (Julian calendar) hard 365 days and 6 hours (every four years, three years are of 365 days and one of 366 days).

   This duration does not exactly match the mean solar year, which is obtained from astronomical observations, shorter than 11 minutes and 14 seconds. As a result, the Julian calendar accumulates one day late each about 128 years.

   Between the year 325, when the Council of Nicaea established the rule for the calculation of Easter, and the year 1582 had already accumulated a difference of about 10 days.  For example, the spring, according to astronomical observations, does not start on March 21, but on 11 March.

   Thus the Passover, that had to fall on the first Sunday after the full moon of spring, often came to fall into the wrong date.

   Pope Gregory XIII realized that Easter, at this rate, it would have to be celebrated in the summer. He decided it was time to address the issue. To reform the Julian calendar, Pope Gregory XIII appointed a commission of experts chaired by Bavarian mathematician Christopher Clavius, a Jesuit. At work gave a decisive contribution the Calabrian doctor Luigi Lilio, the Sicilian,  mathematician and astronomer, Giuseppe Scala and the mathematician  Ignazio Danti of Perugia. To change the Julian calendar was used measurements of the astronomer Nicolaus Copernicus, published in 1543 (the year of his death).

   It was therefore decided to:

1. recover the lost days, in order to realign the start date of the season with what we had in the year 325;
2. change the average length of the year, in order to prevent the recurrence of this problem.

   The Gregorian calendar immediately entered into force: October 15 1582.

   To recover the lost ten days, it was decided that the day after October 4 1582, was October 15.

   In addition, to avoid disruption of the week, it was agreed that October 15 was a Friday and, the previous day, October 4 , was a Thursday.

   To change the mean duration of the year, was changed the rule that decides leap years: according to the new rule, the years whose numbering is multiple of 100 are leap years only if it is even multiple of 400: that is to say, are bissextile the years 1600, 2000, 2400 ..., but are not bissextile the years 1700, 1800, 1900, 2100, 2200, 2300, ... . All other years whose number is a multiple of 4 are leap years. In the centuries preceding remains valid to the Julian calendar then the years 1500, 1400, 1300, ... are bissextile years.

   So in the Gregorian calendar there are 97 bissextile years every 400 years ... while in the Julian calendar there are 100 bissextile years every 400 years. The average Gregorian year is therefore "3/400 of day - 10 minutes and 48 seconds" shortest of the year Julian: the difference from the calendar year is only of 26 seconds (in excess). This discrepancy is equivalent to about one day every 3323 years.

   So, in the year 4905 (=1582+3323) should be abolished one day.

   The Gregorian calendar gains a day compared to Julian calendar every time "jumps" the bissextile year: thus the difference, which was 10 days in the year 1582, has become of 11 days in the year 1700, of 12 days in the year 1800 and 13 days in the year 1900, in the year 2100 will become of 14 days, in the year 2200 of 15 days, and so on.