Double your dating method
It is only from about the year 679 that we are able to appeal to English charters of indisputable authenticity.
It adds about thirty-eight years to the ordinary numbering of the Christian Era.It is in any case certain that neither in the papal chancery nor in that of the Western Empire was the system introduced until considerably later.In the empire it only became general in the latter part of the ninth century, while although it occurs occasionally in papal documents of the time of John XIII (965-972), it was not the rule before the twelfth century."Those who have seen it say that the document which John brought does not bear the day or the indiction . Even down to the beginning of the twelfth century not only royal and imperial letters but even charters (), properly so called, were occasionally through the carelessness of officials sent out without a date.(Bresslau, Handbuch, I, 891.) In this matter the Italian chancery officials seem to have been much more careful than those of the rest of Europe.But even the bulls of such a pontiff as Innocent III are not unfrequently at fault, and as Léopold Delisle has shown, an erroneous calculation of the indiction may be perpetuated through a whole series of authentic documents (Bib. The point of main interest in this connection is to determine the source and period of the introduction of our present system of dating by the Christian Era.
Although, as explained in the article GENERAL CHRONOLOGY, the monk known as Dionysius Exiguus, when resident in Rome, c.
that they must bear upon them the indication of the day and year when they were delivered, may be traced back to the time of Constantine.
In the course of the Middle Ages this principle was generally admitted, and we find, for example, that at Cologne in the twelfth century the validity of a certain instrument was contested because it lacked a date. now the Roman decrees lay down that letters which lack the day and the indiction have no binding force." (Westdeutsche Zeitschrift für Geschichte, I, 377.) But although this principle was recognized in theory it was not always carried out in practice.
In the intervening year which breaks this series into two parts falls the death of Bede A. 735." Very noteworthy is the decree of an English synod held in 816, wherein it is prescribed that the bishop shall put the acts of the synod into writing and date them by the Era of the Incarnation.
This points no doubt to a time "when ecclesiastics knew the era well enough but had not yet acquired the punctual habit of using it".
But for the dating of papal documents and for the so-called "double date" see the article BULLS AND BRIEFS.