The chronology of his writings is difficult to fix with certainty. It is in part determined by the Montanistic views that are set forth in some of them, by the author's own allusions to "this" writing, or "that," as antedating others (cf. Harnack, "Litteratur" ii.260–262), and by definite historic data (e.g., the reference to the death of Septimius Severus, "Ad Scapulam", iv). In his work against Marcion, which he calls his third composition on the Marcionite heresy, he gives its date as the fifteenth year of the reign of Severus ("Adv. Marcionem", i.1, 15) – which would be approximately 208.