Copts preserved the chronological system of the first Christians

from the series of short articles about calendars

During the reign (284-305 AD) of Emperor Diocletian (244-311 AD). The reforms of Diocletian (Gaius Aurelius Valerius Diocletianus Augustus)'s reforms affected seriously the structure of Roman imperial government and helped stabilize the empire economically and militarily. He was the man, who enabled the empire to remain the main force in the Western European and Hellenic region for another 150 years, avoiding a nearly certain collapse in the time when he took matters in hand. On the other hand Diocletian was in charge for religious persecutions against Manicheans and Christians, as he reasonably considered then as a threat for the cohesion of his empire. Probably, Galerius, the other governor in the empire, pushed him for more drastic actions than those he firstly suggested. The great and the last persecution against Christians was held in 303 AD and the focus of the operations was Nicomedia. The persecution took tremendous dimension and Diocletian was presented in the stories of Christians as the Devil himself. A number of adherents of the new religion suffered or died in incredible torments, while a great part of them were compelled to resign their religion. In the memory of these Martyrs, Christians decided to adopt a new chronological system, starting on the year of the accession of Diocletian (284 AD).
The year number 284 AD was not used in that period, it is an anachronistic year numbering, as we extend the contemporary chronological system to the past. The AD system was introduced later in 525 AD, by the Skythian monk Dionysius Exiguus (an info article about this transition will be given in the future in the frame of this series of articles). The chronological system used in the Roman Empire was the date/month according to the Julian Calendar, while the year numbering was referring to the year of the establishment of Rome (753 BC). Therefore 284 AD was referred as 1037 AUC (Anno Urbis Conditae). The Christians defined the year 284 AD as the year 1 A.M. (Anno Martyri/ Era of the Martyrs) or year 1 Anno Diocletiani (from the reign of Diocletian).
Copts are the Miaphysites Christians of Egypt. In 451 AD, following the Council of Chalcedon, the Church of Alexandria was divided into two branches: (1) those who accepted the terms of the Council became known as Chalcedonians or Melkites. (2) those who did not adhere to the Council's terms and were called by Orthodox Chalcedoneans as non-Chalcedonians or Monophysites and later Jacobites after Jacob Baradaeus. The non-Chalcedonians, however, rejected the term Monophysites as erroneous and referred to themselves as Miaphysites. The majority of the Egyptians belonged to the Miaphysite branch, which led to various persecution by the Byzantine authorities in Egypt.
Copts were those Christians, who preserved the chronological numbering set by the first Christians. The beginning of the year is not 1st of January, but 29th /30th of August (30th in the year before the Gregorian leap year) in the Old Style Julian Calendar, which has been moved to 11th / 12th of September in the current New Style Gregorian Calendar. This is the reason for the correspondence of each Gregorian year to a couple of consecutive Coptic years. The current year 2018 AD is numbered in the Coptic Calendar as 1734/ 1735 Anno Martyri/ Anno Diocletiani. The names of the months come from old Egyptian gods.
The 1st month is called Thout from the name of Thoth, god of Wisdom and Science (11th of September 2018 AD is 1st of Thout 1735 Anno Martyri).
The 2nd month is called Paopi from the name of Hapi, god of the Nile (Vegetation).
The 3rd month is called Hathor from the name of Hathor, goddess of beauty and love.
The 4th month is called Koiak from the name of Ka Ha Ka, the god of Good, the sacred Apis Bull.
The 5th month is called Tobi.
The 6th month is called Meshir from the name of Mechir, god of wind.
The 7th month is called Paremhat.
The 8th month is called Parmouti.
The 9th month is called Pashons.
The 10th month is called Paoni.
The 11th month is called Epip, from the name of Apida, the serpent that Horus, son of Osiris, killed.
The 12th month is called Mesori, from the name of Mesori, the birth of Sun.
The 13th month is called Pi Kogi Enavot, the last little month of 5 or 6 days, in the case of normal/ leap Coptic year respectively.