The Era of the Martyrs (Latin: anno martyrum, abbreviated AM), also known as the Diocletian era (Latin: anno Diocletiani), is a method of numbering years used by the Church of Alexandria beginning in the 4th century AD and by the Coptic Orthodox Church of Alexandria from the 5th century to the present. Western Christians were aware of it but did not use it. It was named for the Roman Emperor Diocletian who instigated the last major persecution against Christians in the Empire. Diocletian began his reign 20 November 284 during the Alexandrian year that began on 1 Thoth, the Egyptian New Year, or 29 August 284, so that date was used as the epoch: year one of the Diocletian era began on that date. This era was used to number the year in Easter tables produced by the Church of Alexandria.
When Dionysius Exiguus continued those tables for an additional 95 years, he replaced the anno Diocletiani era with his anno Domini era because he did not wish to continue the memory of a tyrant who persecuted Christians. The anno Domini era became dominant in the Latin West but was not used in the Greek East until modern times.
The anno Diocletiani era was not the only one used by early Christians. Most Roman Christians, like the pagan Romans before them, designated their years by naming the two consuls who held office that year. The Romans also used the ab urbe condita (AUC) era. Its name is Latin for "from the founding of the City (Rome)." However, the AUC era was hardly ever used outside historical treatises.
Eras that began at Creation, called anno Mundi eras, became the dominant method of numbering years in the East until modern times, such as in the Byzantine calendar. Annianus of Alexandria, a monk who flourished at the beginning of the 5th century, placed the epoch of his world era on 25 March 5492 BC by counting back eleven 532-year paschal cycles from anno Diocletiani 77, itself four 19-year lunar cycles after anno Diocletiani 1. Regarded as a civil rather than a religious era, it began on the first day of the Alexandrian year, 29 August 5493 BC. This Alexandrian era was the preferred era used by Byzantine Christians such as Maximus the Confessor until the Byzantine era, having an epoch of 1 September 5509 BC, became dominant in the 10th century. Both eras used a version of dating Creation based on the Septuagint.