The phrase s.n.t. qaḍāʾ al-muʾminīn, used in several seventh-century Arabic papyri, has been subject to varying interpretations for several years. Yūsuf Rāġib considers it as an "era (sanat) of the believers' jurisdiction," while Jelle Bruning interprets it as a "legal sunna." This chapter uses new papyrological data to reexamine these competing explanations. This expression appears so far only in documents relating to debts, some of which were subject to institutional registration in Fusṭāṭ. The new documents we are editing here, as well as minor paleographic evidence, suggest that the reading sanat qaḍāʾ al-muʾminīn initially proposed by Rāġib is the most convincing, and that it refers to a calendar. Among several hypotheses, we argue that the term qaḍāʾ should be understood as a "decree" and refers to the sovereignty of the "believers," semantically associated with God's decree. This specification might have been particularly important for acknowledgment of debts in order to comply with the Qurʾanic injunction (2:282) to record the debts in writing by precisely defining their deadline. We hypothesize that this was a name of the official imperial calendar, which originally may not have referred to the Muḥammad's exodus to Medina, but rather to the affirmation of his sovereignty following the treaty of al-Ḥudaybiyya.