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.
Gregory the Great the Dionysian Era is not adopted.It was the pope's habit to date his letters by the regnal years of the emperor and letters so dated may be seen in Bede's "Ecclesiastical History", just as they were copied from the Roman archives.Apparently it was the Englishman Bede himself who was the first to bring the Dionysian system into general use, for it was through him that it was adopted in literature, having been employed systematically not only in his "De Temporum Ratione" but especially in his "Ecclesiastical History".The principle that imperial decrees and charters must be "dated" as a condition of validity, i.e.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.