Saying · 17 AH (c. 638 CE)
How are we going to decide this dispute?
Said after two men disputed over when a debt of money was due, unable to agree whether "by Sha'ban" meant that year or the next.
The Arabs before Islam had no unified calendar; each tribe dated events from its own local milestones, and the practice of al-Nasi (swapping the order of the four sacred months to suit the powerful tribes' convenience) had thrown the months out of their proper order [Qur'an, 9:37]. Most likely in the 17th year of the Hijrah, Umar ibn al-Khattab RA was presented with a dispute between two men over when a debt of money was due by "Sha'ban", each understood the term differently since there was no standard calendar, and around the same time received a letter, reportedly from Abu Musa al-Ash'ari RA or one of his governors, asking for a way to date instructions so they were not confused between one year and the next. Umar RA gathered the sahaba to decide on a calendar; a suggestion to adopt the Roman or Persian calendars was rejected, and the sahaba differed over which milestone should mark year one (the Prophet ﷺ's birth, his death, or the Battle of Badr were all proposed) until Ali ibn Abi Talib RA suggested the year of the Hijrah, which Umar RA accepted as the wisest opinion. They then differed over which month should open the year, until Uthman ibn Affan RA settled on Muharram.
This established 1 AH as the first year of the Islamic calendar (with no year 0) and Muharram as its first month, reflecting the sahaba's recognition that the Hijrah was the one decisive turning point that changed the Muslims from being humiliated to being honored. Later scholars have tried to find a Qur'anic allusion to this in [9:108], but Allah knows best.
Other events recorded in 17 AH. Their order within the year is not established; use the storyline above for the sequence.
How are we going to decide this dispute?
Said after two men disputed over when a debt of money was due, unable to agree whether "by Sha'ban" meant that year or the next.
O Umar, sometimes you tell us to do something by a particular month, but we do not know if you mean the same year's month or the next year's. Find a way to tell us.
Which year should we begin with?
Asked of the gathered sahaba after Umar ibn al-Khattab called them together to establish a calendar of their own, rather than adopt that of the Romans or Persians.
The year of the Hijrah shall be the first year of the calendar, because this was the one decisive thing that changed the Muslims from being humiliated to being honored.
This is the wisest opinion.
It shall be Muharram.
Said when the sahaba then differed over which month should open the new calendar.
“Indeed, the number of months with Allah is 12 [lunar] months in the register of Allah [from] the day He created the heavens and the earth; of these, four are sacred.”
This ayah (Q9:36), on the twelve months and the four sacred ones, is the Qur'anic basis for the lunar calendar structure established at this event -- the very next verse (9:37) is cited in the event's own description regarding al-Nasi.