The Prophet's companions did not take as a starting date for the Muslim calendar, the day, the Prophet (ﷺ) had been sent as an Apostle or the day of his death, but the day of his arrival at Medina.
Sahih al-Bukhari Book 63, Hadith 159
← Previous Hadith I heard `Umar bin `Abdul-Aziz asking As-Sa'ib, the nephew of An-Nimr. "What have you heard about residing in Mecca?" The other said, "I heard Al-Ala bin Al-Hadrami saying, Allah's Messenger (ﷺ) said: An Emigrant is allowed to stay in Mecca for three days after departing from Mina (i.e. after performing all the ceremonies of Hajj)"

Sahih al-Bukhari Book 63, Hadith 158 Next Hadith → Originally, two rak`at were prescribed in every prayer. When the Prophet (ﷺ) migrated (to Medina) four rak`at were enjoined, while the journey prayer remained unchanged(i.e. two rak`at).

Sahih al-Bukhari Book 63, Hadith 160