Bukhari

Hadith Commentary:

Ibn Jawzi rahimahullah said: When the population of the world increased, the reckoning of history began from the time of Adam. Now, from Adam to the flood of Nuh, there is one era; from the flood of Nuh to the casting of Ibrahim alayhis salam into the fire, the second; and from that time to Yusuf alayhis salam, the third.

From there to the departure of Musa alayhis salam from Egypt is the fourth.

From there to Dawud alayhis salam is the fifth.

From there to Sulayman alayhis salam is the sixth, and from there to Isa alayhis salam is the seventh; and the history of the Muslims begins from the Hijrah of the Prophet sallallahu alayhi wa sallam. Although the Hijrah took place in Rabi' al-Awwal, the beginning of the year was set from Muharram.

The Jews reckon their history from the destruction of Bayt al-Maqdis, and the Christians from the ascension of Masih alayhis salam.

Hadith Commentary:
➊
Umar radi Allahu anhu initiated the Islamic calendar four years after his caliphate began, when Abu Musa Ash'ari radi Allahu anhu wrote to him saying that your letters reach us without any dates on them. Please write dates so that it is easier to keep track of events. Therefore, Umar radi Allahu anhu started the calendar. When he intended to begin the calendar, he gathered the noble Companions radi Allahu anhum ajma'in and consulted them. Some suggested starting the calendar from the death of the Messenger of Allah sallallahu alayhi wa sallam. Some advised beginning from the Prophethood (bi'thah), and some suggested starting from the birth of the Messenger of Allah sallallahu alayhi wa sallam. Ali radi Allahu anhu said: Start the calendar from the Hijrah, for the Hijrah is what distinguishes between truth and falsehood. Thus, consensus was reached on his suggestion, and the first month of the year was set as Muharram, even though the arrival of the Prophet sallallahu alayhi wa sallam in Madinah Tayyibah occurred in the month of Rabi' al-Awwal. However, the year was started from Muharram because preparations for the Hijrah had begun in Muharram, and the second pledge of Aqabah had taken place during Hajj. Moreover, this month is when the pilgrims return.

➋
It should be noted that this decision was finalized in the sixteenth or seventeenth year after the Hijrah. (Fath al-Bari: 7/336)

➌
The reality is that there were four possible dates for determining the Islamic calendar: the day of birth, the day of Prophethood (bi'thah), the day of death, and the Hijrah. There is much disagreement regarding the day of birth and the day of Prophethood. The day of death is a day of sorrow and regret, as it revives the memory of separation from the Messenger of Allah sallallahu alayhi wa sallam. Therefore, the Islamic year was started from the Hijrah, and this was deduced from the following noble verse: "A mosque founded on piety from the first day..." (al-Tawbah: 9/108) The foundation of the mosque was laid on the day the Messenger of Allah sallallahu alayhi wa sallam and his noble Companions radi Allahu anhum ajma'in entered Madinah.

➍
It should be noted that among the Arabs, the calendar is based on the lunar year; the solar year is not considered. In this reckoning, the night comes first and the day follows, because the moon appears at night first, and then the day begins. In any case, the people of Islam began their calendar from the first of Muharram and deduced it from the Qur'anic verse (min awwali yawm) because at that time the progress of Islam began, and they had the opportunity to propagate Islam in peace and security. Also, the foundation of Masjid Quba was laid.

➎
The Jews began their calendar from the destruction of Bayt al-Maqdis, and the Christians from the ascension of 'Isa alayhis salam. Furthermore, the first of Muharram is the beginning of the Hijri year, not its completion. This means that today is the beginning of 1st Muharram 1429, not the completion of 1429, as is commonly thought. And Allah knows best.