The numbering of years and calendrical reckoning has always been a controversial topic. The civil calendar used in most countries is also known as the Gregorian calendar. The initial epoch (first year) of the Common Era (CE) is 1 CE also known as AD 1. The first century comprises the years AD 1 through AD 100. The second century began with AD 101 and continued through AD 200. By extrapolation, the 20th century comprises the years AD 1901-2000. Therefore, the 21st century began on 1 January 2001 and will continue through 31 December 2100.
Similarly, the 1st millennium comprised the years AD 1-1000. The 2nd millennium comprised the years AD 1001-2000. The 3rd millennium began with AD 2001 and will continue through AD 3000.
Over recorded history, many initial epochs have been used for calendrical reckoning. Frequently, years were counted from the ascension of a ruler. For a calendrical epoch to be useful, however, it must be tied to a sequence of recorded historical events. The initial year of the Common Era was chosen to be first year to commence after the birth of Christ. This is the initial epoch of the Julian and Gregorian calendars. This epoch was established by the 6th century scholar Dionysius Exiguus who was compiling a table of dates of Easter. Dionysius followed a previous precedent, extending an existing table (by Cyrillus) covering the period 228-247. This table was reckoned from the beginning of the reign of Emperor Diocletian. However, Dionysius did not want his Easter table "to perpetuate the memory of an impious persecutor of the Church, but preferred to count and denote the years from the Incarnation of our Lord Jesus Christ." To accomplish this, he designated the years of his table Anni Domini Nostri Jesu Christi 532-550. Thus, Dionysius' Anno Domini 532 is equivalent to Anno Diocletiani 248, establishing a correspondence between the new Christian Era and an existing system associated with historical records. Dionysius did not, however, establish an accurate date for the birth of Christ. Scholars generally believe that Christ was born a few years before AD 1, but the records are too sketchy to allow a definitive dating.
How are dates prior to an initial epoch recorded? Today it appears obvious that a year designated 1 would be preceded by year 0, which would be preceded by year -1, etc. But the concept of both 0 and negative numbers did not come into common use in Europe until the 16th century. Even then, these concepts were initially only of interest to mathematicians. Their application to chronological problems did not occur for two more centuries. Instead, years were counted using a succession of initial epochs. Even as Dionysius' practice of dating from the Incarnation became common in ecclesiastical writings of the middle ages, traditional dating practices continued for civil purposes.
In the 16th century, Joseph Justus Scaliger tried to resolve the patchwork historical eras by placing everything on a single system. Not being ready to deal with negative year counts, he sought an initial epoch in prior to any historical record. His approach was numerological and utilized three calendrical cycles:
- the 28-year solar cycle,
- the 19-year Golden Number cycle, and
- the 15-year indiction cycle.
The solar cycle is the period after which week days and calendar dates repeat in the Julian calendar. The Golden Number cycle is the period after which moon phases repeat (approximately) on the same calendar dates. The indictions originally referred to the periodic reassessment for an agricultural or land tax in late third-century Roman Egypt. Scaliger could characterize a year by the combination of numbers (S,G,I), where S runs from 1 through 28, G from 1 through 19, and I from 1 through 15. A given combination will recur after 7980 (= 28 x 19 x 15) years. He called this a Julian cycle because it was based on the Julian calendar. Scaliger knew that the year of Christ's birth (as determined by Dionysius Exiguus) is characterized by the number 9 of the solar cycle, by Golden Number 1, and by number 3 of the indiction cycle, or (9,1,3). Scaliger chose as this initial epoch the year characterized by (1,1,1) and determined that (9,1,3) was year 4713 of his chronological era. John Herschel later adopted Scaliger's initial epoch for the Julian Day numbers used by astronomers.
We would say that Scaliger's initial epoch was 4713 BC or -4712. In the historical system of dating, AD 1 is preceded by 1 BC. That is, there is no year 0. In the astronomical system, AD 1 is designated +1 and is preceded by year 0. The historical system was introduced in the 16th century. However, the astronomical system was not introduced until the 18th century.
