The present article is devoted to the problem of correlating the events dated according to the Hegira within the first ten years of the Muslim era to the Julian calendar. Before Islam in Mecca as well as, in all probability, in other regions of the Arabian peninsula, a lunisolar calendar with a periodic intercalation of the additional (13th) month was in use. The interdiction to insert that month was unambiguously formulated by the Prophet Muḥammad only in the course of the «Farewell Pilgrimage» at the end of the 10th year of the Hegira (in early March of A. D. 632). However, the Muslim tradition extrapolated that prohibition to the first ten years of the Hegira, so that such an approach proved to be reflected all the synchronistic tables. The greatest Mathematician and Astronomer of the medieval Islam Abū Rayḥān al-Bīrūnī was perfectly aware of that inconsistency and paid attention on it in his Canon of Mas’ūd. Nevertheless modern scholars specialized in early Islam, acknowledging in wo
