*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]
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?
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because remembering it would renew their grief. So therewas no choice
left except the Hijrah. And they choseto regard Muharram as the first
month of the year rather than Rabee' al-Awwal because the plan to
migrate started totake shape in Muharram. The oath of allegiance
(bay'ah) – that was the precursor to the Hijrah – had taken place
during Dhu'l-Hijjah, and the first new moon after the oath of
allegiance and 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
thepeople and asked them what the first day of the calendar should be.
'Ali said: (It should start) fromthe day when the Messenger of Allah
(blessings and peace of Allah be upon him) migrated and left the
landof shirk. So 'Umar did that.End quote.
The one who says that the age of Islam started with the Hijrah is
referringto 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 somethingon 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 onlybegan with the
Hijrah.
As for the beginning of Islam itself and people's awareness of it, we
do not need to point out thatthis took place before thattime. Indeed
the meaningof the word Islam in general includes the religion that
Allah approved for His slaves and with which He sent His Prophets and
Messengers. But this is notwhat we are discussing here.
We do not believe that anyone could imagine that Islam only began with
the Hijrah and ignorethe years of da'wah during which the Prophet
(blessings and peace of Allah be upon him) and those who were with
himin Makkah strove to establish the faith. No onewould say this.
And Allah knows best.