Many medieval and early modern Jewish calendars were based on the assumption that the calendar repeats itself exactly after 247 years. Although this cycle—known as the ʿIggul of R. Naḥshon Gaon—is discussed in many sources, both medieval and modern, its origins remain a mystery. The present article sheds light on the early history of the reiterative Jewish calendar by looking at the oldest 247-year cycles identified to date. Textsf rom the Cairo Genizah demonstrate that the 247-year cycle originated in Babylonia in the middle of the tenth century and was produced by Josiah b. Mevorakh (ibn) al-ʿĀqūlī, previously known from Judeo-Persian calendar treatises. In contrast, a large body of manuscript evidence shows that the attribution of the cycle to R. Naḥshon Gaon (874–882 CE) is not attested before the twelfth century and may be unhistorical. The 247-year cycle may have been proposed as an alternative Jewish calendar that would eliminate the need for calculation and prevent calendar divergence. But at least from the early twelfth century the cycle was seen as a means of setting the standard calendar, even though it is not fully compatible with the latter.