This paper studies how Early Modern Spanish historians confronted the problem of calculating the equivalence between the Christian Era and the Hegira. Chronological polemics concerning the Hegira were deeply embedded in a major historiographical problem, namely the role Islam and al-Andalus played in the history of Spain. Besides the technical issues, chronology is one of the most important ways by which an Islamic Iberian past was integrated in a narrative about national history. Once Islam became a historical actor for Spanish and European historians, rather than just a religion to confront, very important questions were raised: were Arabic sources necessary for the writing of Spanish history? What were these sources, and what was their value? Since al-Andalus was connected with the more general problem of the relationship of ancient Spain with the Orient (and, specifically, with the Biblical Orient), the chronological argument became a major issue in reflections on the limits and possibilities of writing the sacred history of Spain.