In c. 638 (seventeen AH), Abu Musa al-Ash'ari, on the list of officers from the Rashid Caliph Umar (r. 634–644) in Basra, complained regarding the absence of any yrs over the correspondence he gained from Umar, rendering it tricky for him to select which Guidance were most recent. This report persuaded Umar of the need to introduce an period