"The Battle of the Milvian Bridge"

We are now living in the 113th Indiction Cycle. The new year however now begins on 9 September (Gregorian calendar) since there was an 8 day slippage between 1 September 312 AD (Julian calendar) when Constantine's first Indiction Cycle began and the 16th century when Pope Gregory reformed the calendar and eliminated further slippage between solar and lunar years.

On 28 October 312 Constantine (the Great) won control of the Western Roman Empire at the Battle of the Milvian Bridge. Following a practice introduced by Diocletian in 285, Constantine issued tax assessments based on a tax year beginning on 1 September (in Egypt, usually on 1 July). Diocletian used to issue these assessments ('Indictions') in five-year cycles. Constantine the Great, who was first proclaimed Emperor at York in 306, issued his first Indiction to date from 1 September 312, the year in which he invaded Italy to attack Maxentius. Once he defeated his rival, Maxentius, at the Battle of the Milvian Bridge on 28 October 312, his Indiction could be applied throughout the Western Roman Empire. In the subsequent years Constantine consolidated his power and extended his authority to the Eastern Roman Empire with the defeat of Licinius on 19 September 324. He moved his capital to Byzantium, renamed Constantinople or New Rome, which he inaugurated by ploughing its pomerium on 8 November 324. Fifteen years after the start of his first Indiction, on 1 September 327, he issued a new Indiction of fifteen years' duration and the practice developed of dating legal documents in fifteen-year rather than five-year cycles, starting always on 1 September. (Strictly speaking, the term Indiction, with a Roman number appended, refers to the individual years within each fifteen-year cycle and the cycles themselves are not numbered). Since decimals had not been invented, there was no prevailing concept of a decade or a century so multiples of fifteen may have seemed no odder than multiples of ten. The practice of numbering indictions in fifteen-year cycles was continued by Constantine's sons and carried on in Byzantium until its fall in 1453. It was also maintained in the West by Charles the Great and his successors and persisted until 1806 when Napoleon dissolved the Holy Roman Empire.