"The dating of the Islamic years was introduced by Second Caliph Umar Bin Al-Khattab in 638 CE (16 AH) in an attempt to circumvent all the various conflicting dating systems used during his time. After consulting his companions, he set the Hijra - the journey of the Prophet (peace be upon him) from Makkah to Madina - as the most appropriate reference to the new Islamic era. The Hijra, historically speaking, is the central event of early Islam, the turning point in Islamic history that led to the foundation of the first Muslim state.

There was not any Islamic calander during the time of Prophet (peace be upon him)How did people know about ramdaan, hajj and friday prayers?
And what does Quran refers to Ramdan in chapter 2. Name of a month or condition of a season?