Since E.S. Kennedy and David Pingree’s pioneering work on the ʾAbbāsid-era astrologer Māshāʿallāh (fl. late 8th-early 9th cent. CE), it has been accepted that this scholar relied upon a version of the Sasanian astral science handbook known in Arabic as the Zīj al-Shāh for his calculations. This handbook seems to have existed in redactions commissioned by the Sasanian rulers Khusrō I (r. 531-79) and Yazdgerd III (r. 632-51). Kennedy and Pingree maintained that Māshāʿallāh must have relied on the earlier of these two redactions, due to 60-day discrepancies between two Zoroastrian calendrical dates given by Māshāʿallāh in his astrological history, and what they would be according to the Zoroastrian calendar that seems to have been otherwise standard in Māshāʿallāh’s day. For Kennedy and Pingree, this was an unmistakable sign that the version of the Zīj al-Shāh consulted by Māshāʿallāh must have been the one composed in Khusrō I’s reign, because it reflected an old form of the Zoroastrian calendar, in which two months of 30 days each had not yet been intercalated. The historicity of this Sasanian-era intercalation, which Kennedy and Pingree took for granted, has been persuasively disproven by François de Blois, but the relevance of de Blois’ finding for the interpretation of these problematic dates in Māshāʿallāh has not yet been explored. In this paper, I argue that Māshāʿallāh’s dates do not reflect his reliance on any version of the Zīj al-Shāh in particular, or, more to the point, any large-scale intercalation undertaken in the Sasanian period. What ʿallāh’s dates do reflect is his close association with Zoroastrian landowners, and their efforts, as discussed by de Blois, to reform the Zoroastrian calendar according to which the yearly harvest tax was collected. This calendar had become increasingly unmoored from the natural seasons, due to its years of a constant 365 days. While there would have been general agreement that some calendrical reform was called for, Zoroastrians would have favored an intercalation of whole-number months, rather than the finer adjustment favored by the ʾAbbāsid caliphs; this way there would have been minimal disruption to the normal course of Zoroastrian ritual observance, with each day of the month consecrated to its eponymous deity. Māshāʿallāh’s astrological history, then, reflects a fictitious Sasanian precedent, developed for these ʾAbbāsid-era calendrical disputes by his Zoroastrian contemporaries, rather than a survival from some pre-intercalation era of Sasanian history.