It is commonly believed that the Anno Domini calendar, which counts the years from the birth of Christ, was adopted in the Roman Empire immediately after the Christianization of the Empire in the fourth century. But this was not the case. As a matter of fact, the A.D. system was not used at all in Europe until the tenth/eleventh century, and was not widely used until about the fourteenth century.
The Romans of Classical times employed several different dating systems, one of which, Ab Urbe Condita ('from the founding of the city') claimed to count the years from the establishment of Rome by Romulus. Several other calendars, including the 'Alexandrian' were used simultaneously with this. The Alexandrian Calendar dated from the reign of Alexander the Great. All of these systems continued to be used after the Christianization of the Empire, with the addition of one other, based on the Bible. This was the Anno Mundi ('Age of the World') calendar. This latter claimed to date the age of the earth based on the lifespans of biblical Patriarchs encountered in the Book of Genesis and elsewhere in the Hebrew Bible. The age of the earth was of great interest to early Christians, because it was assumed that Christ's return would occur after six thousand years. This latter notion was ultimately based on a statement by Saint Peter that to God "a thousand years is as a day", with the six days of Creation being analagous to the six thousand years of earth history. And, just as Genesis has God resting on the seventh day, it was assumed that the seventh millennium would see the glorious reign of Christ over the world.
So, for early Christians, the important information was not the length of time which had elapsed since the birth of Christ, but the length of time that had elasped since the creation of the world.
The problem with using the Bible to date the earth's antiquity was that Genesis is by no means clear when one generation of Patriarchs ends and another begins. In addition, different versions of the Bible gave different figures. In this way, it was argued by one school of biblical scholarship in the fourth century that the world was 5,000 years old, whereas another and competing school argued that it was 5,300 years old. And other schools gave other dates.
When the Western Empire was abolished in the fifth century, its provinces came to be ruled by various Germanic dynasties. Gaul, for example, was largely under the control of the Franks. Spain was ruled by the Visigoths, whilst Italy came under the dominion of the Ostrogoths and North Africa under that of the Vandals. All of these kingdoms employed differing and often competing calendars, and with the disappearance of the centralized Roman bureaucracy there was little chance of agreement.
The Germanic kings of the West may have controlled the Western provinces, but they were not entirely free of the control of the Empire, which still existed in the East and had its capital in Constantinople. The Germanic kings minted their own coins, but these were invariably emblazoned with the likeness of the Eastern Emperor. The latter may not have possessed the military capacity to compel the obedience of the Germanic kings, but he had the money to hire one king against another, should any of them display too much independence. Only during the sixth century, under Justinian, was there a serious attempt to exert direct control of the West using military force. And it was at that time too that a Germanic king - Theodahat of the Franks - briefly declared himself Emperor of the West and minted coins with his own likeness on them. This latter was viewed by the writer Procopius as an appalling sign of decline and decay.
The Germanic kings then would perhaps have liked to style themselves Western Emperors, but they were rightly wary of Constantinople's reaction should they do so. Things changed however in the first quarter of the seventh century, when Constantinople lost virtually all of her Asian and North African territories to first the Persians and then the Arabs. These events shattered Byzantium's power and made conditions ripe for a revival of the Western Empire under a Germanic king. We are told that this did not occur until the reign of Carl the Great (Charlemagne) in 800 A.D., almost two centuries later, but, if Heribert Illig is correct, it happened much sooner than that.
By the middle of the seventh century, a Frankish king of the eastern Merovingian realms, a man named Otto (Otto I), had achieved dominance throughout most of what we now call France and western Germany. After crushing the Avars (also known as Magyars) at the Battle of Lechfeld, Otto gained control of most of northern and central Italy; and, in 662 (supposedly 962), was crowned Emperor of the West in Rome by Pope John XII. As noted above, Otto was able to do this because by the mid-seventh century the power of Constantinople was broken. The West was now able to detach itself politically and religiously from the East. Yet as a German, Otto felt something of a usurper. He needed a precedent, an earlier German king who had donned the Imperial Purple, and he found this is Theodahat, the Frankish king who, a hundred years earlier, had battled against Justinian and briefly proclaimed himself Emperor. And the figure of Theodahat was somewhat confused with an even earlier and more powerful Germanic king, Theodoric the Ostrogoth, who had ruled the whole of Italy and much of Gaul during the late fifth and early sixth century. Both these kings had described themselves as a "Carl", a Warrior.
Otto I therefore styled himself as the legitimate successor to this earlier Carl and inaugurated a cult of Carl the Great, (Charlemagne) the first Holy Roman Emperor.
Yet this historical appropriation was not without problems. Although Theodahat and Theodoric were powerful kings who had defied the Emperor in the East, neither had been formally crowned Emperor by the Pope. The solution was to elevate "Carl" into a separate king, with his own dynasty. Along with his own dynasty he would also need a couple of centuries in which his dynasty could flourish. This was the solution arrived at by Otto I's grandson Otto III, who in the latter years of the seventh century formally introduced a new calendar in which a space could be found for the Great Carl and his family.
And there was another issue on Otto III's mind. As a deeply religious man he had noted the rapid spread of the Christian faith throughout Europe, which to him was, of course, virtually the whole world. This was in line from a statement in the New Testament to the effect that the Second Coming of Christ would occur when the Gospel had been preached to all nations. Otto III and his learned observers therefore, together with Pope Sylvester, decreed that the third year of his reign would mark the year 6000 of the earth's history and the year 1000 since the birth of Christ. As such, Otto III was placing himself at the dawn of the Millennium, the final thousand years of Christ's glorious reign over the world.
Otto III was able to get away with this calendrical sleight of hand for the simple reason that in those days most people were illiterate and 'History' as we now understand it - namely a body of literature with an agreed chronology and sequence of events - simply did not extist. The few people who could read - mainly clerics - possessed various chronicles and annals written by Greek and Latin authors, but these men did not date the events they described in any agreed chronological sequence. These were instead dated to the reign of the Emperor or King then on the throne, or to the Olympiads, or to the Age of the World as found in the Bible. And of the latter there were many conflicting estimates.
So, in a premodern world, the declaration by Otto III that January 1 of the third year of his reign marked the year 1000 Anno Domini, caused little comment and, fully endorsed by the Pope and the Church, was accepted with little if any resistance.