The Creation has been adopted as an epoch by Christian and Jewish writers, and would have been found very convenient, by doing away with the difficulty and ambiguity of counting before and after any particular date, as is necessary when the era begins at a later period. But, unfortunately, writers are not agreed as to the precise time of commencing. We consider the creation as taking place 4004 years B.C.; but there are about 140 different variations in this respect. The following are those that have been most generally used :—One reason that this dating system is important to fans of textual criticism is that numerous medieval manuscripts can be dated by the above-described technique (see p. xiv here, for example).
The Era of Constantinople. In this era the creation is placed 5508 years B.C. It was used by the Russians until the time of Peter the Great, and is still used in the Greek church. The civil year begins the 1st of September, and the ecclesiastical towards the end of March; the day is not exactly determined. To reduce it to our era, subtract 5508 years from January to August, and 5509 from September to the end.
Era of Antioch, and Era of Alexandria. We place these together, because, although they differed at their formation by ten years, they afterwards coincided. They were both much in use by the early Christian writers attached to the churches of Antioch and Alexandria. In the computation of Alexandria, the creation was considered to be 5502 years before Christ, and, in consequence, the year 1 A.D. was equal to 5503. This computation continued to the year 284 A.D., which was called 5786. In the next year (285 A.D.), which should have been 5787, ten years were discarded, and the date became 5777. This is still used by the Abyssinians. The era of Antioch considered the creation to be 5492 years before Christ, and, therefore, the year 285 A.D. was 5777. As this was equal to the date of Alexandria, the two eras, from this time, were considered as one. Dates of the Alexandrian era are reduced to the Christian era by subtracting 5502 until the year 5786, and after that time by subtracting 5492. In the era of Antioch, 5492 are always subtracted.
The Abyssinian Era. The Abyssinians reckon their years from the creation, which they place in the 5493rd year before our era, on the 29th of August, old style; and their dates will consequently exceed ours by 5492 years, and 125 days; they have 12 months of 30 days each, and 5 days added to the end, called pagomen,from the Greek word παγωμενα(added). Another day is added at the end of every fourth year. To know which year is leap year, divide the date by 4, and if 3 remain, the year will be leap year. It always precedes the Julian leap year by 1 year and 4 months. To reduce Abyssinian time to the Julian year, subtract 5492 years and 125 days. The Abyssinians also use the era of Martyrs, or Diocletian, with the same months as in the above.
The Jewish Era. The Jews usually employed the era of the Seleucides, until the fifteenth century, when a new mode of computing was adopted by them. Some insist strongly on the antiquity of their present era; but it is generally believed not to be more ancient than the century above named. They date from the creation, which they consider to have been 3760 years and 3 months before the commencement of our era. Their year is luni-solari consisting either of 12 or 18 months each, and each month of 29 or 30 days. The civil year commences with or immediately after the new moon following the equinox of autumn. The average length of the year of 12 months is 354 days; but, by varying the length of the months Marchesvan and Chisleu, it may consist of 353 or 355 days also. In the same manner, the year of 13 months may contain 883, 384, or 885 days. In 19 years, 12 years have 12 months each, and 7 years 18 months.
That was long before Ken Ham (1951 - present) or Archbishop James Ussher (1581 – 1656) were ever a twinkle in their fathers' eyes. For those hopelessly stupid, deluded, or deceived (is there some other option?) people who insist that assigning a young age to the Earth was an invention of the Seventh Day Adventists, keep in mind that these manuscripts go back well before the SDA movement started in 1863 or Ellen G. White (1827 – 1915).