Read the following at your normal rate of speed. Then if you can explain it to your spouse so he or she fully understands, you will both know about everything there is to know.
Each year The Old Farmer's Almanac carries a listing of Chronological Eras, i.e., calendars calculated from a starting point in time different from the one that assumes the year beginning January 1 to be the 1987th from the birth of Jesus Christ. That figure is almost certainly in error, being from two to four years too low, depending on the correct date from the death of King Herod, who was reigning at the time. The point is still unresolved with exactness 14 centuries after an Italian monk calculated it from the best data available to him, his probably erroneous result having in the intervening span become sanctified by usage.
Much greater uncertainty attaches to many of the other Eras in The Old Farmer's Almanac annual listing. Two—the Byzantine Era and the Jewish Era (A.M., Anno Mundi, Latin for “year of the world”), still in use for the religious calendars of the Orthodox (Greek) Christian and Hebrew faiths—start their count from the creation of the world as narrated 10 the Book of Genesis, but differ by 1748 years (5509 B.C. and 3761 B.C., respectively) in their calculation of the date. (Neither agrees with the 4004 B.C. figure arrived at by the 17th-century Protestant Archbishop Ussher and generally accepted by American fundamentalist sects.)
The Japanese Era uses a more localized starting point—the legendary creation of the Japanese islands by the Sun Goddess in 660 B.C.
Against these chronologies, which have to be accepted on faith rather than on any hard evidence, the remaining seven in the listing can be related with reasonable accuracy to known historical events. The Islamic Era of the Hegira (“flight”), which starts from Mohammed’s exile in our A.D. 622 to Medina, where he received his revelation, is unquestionably correct in its year count; but unfortunately it is computed by lunar years, which are shorter than our Western solar years, and thus it is difficult to correlate with our own calendar. If the Moslems used solar years as we do, 1987 would be their year 1365 instead of 1408.
In India the traditional calendar is based on the Era of the Sakas, a Central Asian tribe of invaders who over-ran northern India in the first century A.D. The Sakas are said to have started ruling India in our A.D. 78, and the era starts from that date; but whether it is exact or only approximate, historians are so far unable to determine.
For something like a thousand years, until the Christian Era started to be used in the sixth century A.D., and well beyond that in some areas, the Western world counted its years from the traditional founding of Rome in 753 B.C. (A.U.C.—ab urbe condita, Latin for “from the founding of the city”). No historical record of that event has survived and the date may be only a wild guess—but the Roman Era which was based on it did duty long and well, and out of respect to it, almanacs (including the OFA) still tell us what year it would be if it were still in use (1987 = 2740).
None of these eras above—except obviously the Christian one, and even that is now starting to be called by the neutral appellation of Common Era—has the name of an individual person; but three that are still to be told about do have that distinction. The oldest of the three is the Era of Nabonassar, which by recent recomputation by historians is now counted to start from 749 B.C. instead of 747 B.C. as stated in older almanacs; and oddly, Nabonassar himself—one of the most insignificant of rulers—had nothing whatever to do with initiating the era.
He had been dead for around nine centuries when the great classical astronomer, Claudius Ptolemy, decided about A.D. 150 to compile dated lists of the Babylonian, Persian, Greek, and Roman kings and rulers. He went as far back as he could locate any references to them, for the purpose of providing cross-checks on the ancient astronomical observation of which he was making a study. The resulting “Canon of the Kings,” as it is called, has been ever since an invaluable resource for historians. The oldest dates in it are those in the Babylonian king list, which commences with Nabonassar in a year that Ptolemy calculated was six years later than the founding of Rome, i.e. 747 B.C., but which is now believed to have been actually 749 B.C.
Nabonassar was a mere puppet-king of Babylonia under the Assyrian conqueror Tiglath-Pileser IV; the king list commences with him simply because there is a gap of several generations before, during which Babylonia fell into general decline and records were either lost or not even kept. Later, Babylonia revived and became great again, until it fell to the Persians two centuries down the line; and for all that stretch the list of kings was maintained from Nabonassar on, providing a continuous link from them to the Persians, and from the latter to the Greeks and Romans. That is why the Era of Nabonassar, with its proven correlations, continues to serve astronomers and historians.
The second Era named for an individual is the Seleucid (or Grecian), which can be fixed to an exact year - 312 B.C., though there is some question whether it commenced in September or October. It takes its name from Seleucus Nicator (i.e., the Vic-torious), one of the generals of Alexander the Great. After that great conqueror’s death in 323 B.C., his army chiefs contended over who should inherit and rule the Middle Eastern lands he had wrested from the Persian Empire. Seleucus, the most energetic and enterprising of them, and a bold and daring adventurer, seized Babylonia with a small band of followers in the late summer of 312 B.c.and established there the kingdom that by his death 30 years later had become the great Seleucid Empire, stretching from Turkey to India. The exploit that laid its foundation caught the imagination of the peoples of the Middle East so wholly and swiftly that in a few months they started using it as the Year 1 of a new Era, which even yet is used in some Greek communities in the Levant.
The last of the three is the Era of Diocletian, named for the Roman Emperor who—somewhat on the model of Seleucus—emerged from obscurity as an army commander to proclaim himself supreme ruler of the Roman Empire in A.D. 284 and proceeded to dispose quickly of all competitors and to institute a series of thoroughgoing reforms to prop up the Empire’s disintegrating fabric. He also initiated the last great persecution of Christians; and both for this and for his renovation of the Empire, his reign came to be regarded as a break with the past and the starting point of a new computation of time. (The count was actually started, for reasons not very clear, from 283, the year before he seized power.) Except in the Middle East, the usage of the Era of Diocletian was, however, never very widespread, and it survives now only as a historical curiosity.
Finally, there is the Chinese Era. Its beginning, 2637 B.C., is based on a legendary date—the founding of the Chinese Empire by the equally legendary emperor, Fo-Hi. However, archaeological evidence indicates that the empire was founded by the Shang Dynasty, approximately 1700 B.c. As with the Byzantine and Hebrew eras, the story is apocryphal—the Chinese version of the creation of the world.