*1] 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?

*1]
I hope insha allah this question reaches u in good health. My question
is I have noticed when a non muslim asks about the age islam after
prophet hood we as Muslims answer only withthe years after hijrah, my
question is why do we always leave out the first 13 years of prophet
hood before hijrah. I know the year of Hijrah was a great year yet we
all know that prophet hood started 13 years before this so when
answering why dont we mention this like now we are 1433 AH yet when
answering why dont we say islam after prophet hood is 1446 adding the
13 years before the period. insha allah you can explain why we dont
add the first 13 years into our answer.
Praise be to Allaah.
No doubt the years that the Prophet (blessings and peace of Allah be
upon him) spent in Makkah before the Hijrah (migration to Madinah),
when he was calling people to the path of his Lord, enduring
persecution and patiently putting up with the annoyance and
accusations of the foolish, could is part of the age of Islam; indeed
they are among the greatest years of Islam because during this time
the Prophet (blessings and peace of Allah be upon him) was relying
completely upon his Lord, thinking positively of Him, and patiently
bearing persecution for His sake.
This is something that no wise person would doubt and no one would
deny it at all, whether he is Muslim or otherwise.
But the reason why the people adopted the Hijri calendar as a means of
defining the year in whicha particular event took place, which is
something that people need to do, is that this date is the one that
the Sahaabah unanimously agreed to take as the beginning of their
calendar. This decision was made at the time of 'Umar ibn al-Khattaab
(may Allah be pleased with him), because it is the actual date of the
founding of the Islamic state, when the Prophet (blessings and peace
of Allah be upon him) and migrated and settled in Madinah, and the
people gathered around him and supported him, and he built the mosque,
and other events that followed the Hijrah. So the features of the
Islamicstate began to develop and it took on a clear form
geographically, socially, militarily and politically. Before that the
Muslims did not have a state or any political system to unite them.
The Sahaabah (may Allah be pleased with them) reached a unanimous
decision in 16 AH – or, it was said, 17 AH or 18 AH – during 'Umar's
caliphate,to make the Islamic calendar begin with the year in which
the Hijrah occurred. That was because a case was referred to Ameer
al-Mu'mineen 'Umar (may Allah be pleased with him) concerning a
contract that one man had with another, saying that what he owed him
became due in Sha'baan.
'Umar said; Which Sha'baan? The Sha'baan ofthis year we are in now
orof last year or of next year? Then he summonedthe Sahaabah and
consulted them about adopting a calendar from which they could
determine when debts became due and the like.
Someone suggested adopting the Persian calendar, but he did not like
that. Someone else suggested adopting the Byzantine calendar, but he
did not like that. Others suggested dating itfrom the birth of the
Messenger of Allah (blessings and peace of Allah be upon him), or from
the beginning of his mission, or from his migration (Hijrah) or
fromhis death.
'Umar (may Allah be pleased with him) was inclined to choose the date
of the Hijrah becauseit is known when it occurred, and they agreedwith
him.
The point is that they made the beginning of the Islamic calendar the
year of the Hijrah, and they made the first monthof the Islamic year
Muharram, as was narrated from them. This is the view of the
majorityof imams, so that people could conduct their business on that
basis, with no confusion.
See: al-Bidaayah wa'n-Nihaayah, 3/251-253
Al-Bukhaari narrated in his Saheeh (3934) that Sahl ibn Sa'd said:
They did not start the calendar from the beginning of theProphet's
mission or from his death; they only started it from the time ofhis
arrival in Madinah.
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
The incidents that are connected to the life of the Prophet (blessings
and peace of Allah be upon him) and that could have been taken as the
start of the calendar are four: his birth, the start of his mission,
his migration (Hijrah) and his death. They thought it was best to
start the calendar from the Hijrah, because in the case of his birth
and the start of his mission, there would be uncertainty with regard
to the exact year. As for the time of hisdeath, they chose not to use
it because remembering it would renew their grief. So therewas no
choice left except the Hijrah. :->