Its Julian equivalent was April 19[2] but it is sometimes mistakenly placed on July 16. The error derives from the tabular Islamic calendar which was devised by later Islamic astronomers. This reckons time backwards according to the lunar calendar, which causes it to miss the three intercalary months (about 88 days) added to the then-lunisolar calendar between the time of the Hijra and AH 10, when Muhammad is recorded as having received a revelation prohibiting their use.[3]