It is unlikely that we have an original copy of this work, so dating its writing is difficult - not least because the Historia is made up of older material, and the currently extant manuscripts have obviously be derived from a mix of manuscripts we don't have. Each has earlier and/or later material. Morris* suggests between 796 CE, which is the last date quoted in the section known as the Northern History, and 806 CE which is suggested by Manuscript 'C' as the date of its original. However, it is likely that the two oldest examples are in 'A' and the 'Vatican'. 'A' gives a complex calculation from the birth of Jesus, through various other dates, to the date of origin, which suggests 829 CE* while the Vatican is probably from 944 CE* This is not to say the manuscripts themselves are this age, just the text in them, though the Vatican manuscript was dated by Gunn and the librarians of the Vatican on the basis of caligraphy to the 10th Century*. However, overall, dates claimed by the manuscripts vary from 796 CE to 994 CE*.