1a] Why do we count the age of Islam from the beginning of the Hijrah and not from the beginning of the revelation and the call?

1a]
to start the calendar from the Hijrah, because in the case of his birth and the start of his mission, there would be uncertainty withregard to the exact year. As for the time of his death, they chose not to use it because remembering it would renew their grief. So there was no choice left except the Hijrah. And they chose to regard Muharram as thefirst month of the year rather than Rabee‘ al-Awwal because the planto migrate started to take shape in Muharram. The oath of allegiance (bay‘ah) – that was the precursor tothe Hijrah – had taken place during Dhu’l-Hijjah, and the first new moon after the oath of allegianceand the decision to migrate was that of Muharram. So it was appropriate to make it the first month of the year. This is the best explanation I have come across as to why the year starts with Muharram.
Al-Haakim narrated that Sa‘eed ibn al-Musayyab said: ‘Umar assembled the people and asked them what the first day of the calendar should be. ‘Ali said: (It should start) from the day when the Messenger of Allah (blessings and peace of Allah be upon him) migrated and left the land of shirk. So ‘Umar did that. End quote.
The one who says that the age of Islam started with the Hijrah is referring to the calendar and what the people agreed upon of creating a system so that people could know the dates of events and define the times of contracts, the dates of visits by delegations, and the like. This is something on which the people agreed during the caliphate of ‘Umar (may Allah be pleased with him) and it has remained so up until the present day. The start of this calendar marks, as ‘Umar intended, the establishment of the (Islamic) state, which only began with the Hijrah.
As for the beginning of Islam itself and people’s awareness of it, we do not need to point out that this took place before that time. Indeed the meaning of the word Islam in general includes the religion that Allah approved for His slaves and with which He sent HisProphets and Messengers. But this is not what we arediscussing here.
We do not believe that anyone could imagine thatIslam only began with the Hijrah and ignore the yearsof da‘wah during which the Prophet (blessings andpeace of Allah be upon him) and those who were with him in Makkah stroveto establish the faith. No one would say this.
And Allah knows best.
/ - - - :-> Transtors: 1.http://free-translation.imtranslator.net/lowres.asp 2.http://translate.google.com/m?twu=1&hl=en&vi=m&sl=auto&tl=en