In the West the birth of Christ marks the beginning of the "AD period". Anything before that falls under "BC". Whilst the Hijrah calendar started when Muhammad migrated from Mecca to Medina.
Now say if Islamic scholars or historians narrated the life of Jesus or Muhammad's life prior to his migration in detail, how would they date certain events such as their birth by using the Islamic/Hijrah calendar?
I'm completley aware that the Gregorian calendar and the Islamic calendar do not correspond with eachother since the former is a solar calendar, and the latter is a lunar calendar. So that means that Ramadhan may fall on the (Gregorian) summer, fall, spring, or winter seasons.
In case you're wondering why I'm not asking this in r/islam, it's because of their strict flair rule that won't let me post.
I'm on my phone, so I can't really go into details.
As far as I know, they just count back. You may be familiar with the historian al-Tabari, who wrote what can basically be considered a history of the world (starting with Adam and Eve, then the Persian kings, Alexander, the Romans, then the Sassanians, Muhammad, all the way up to basically his own period). In the pre-hijri period he counts down, organizing his annals per year (in the year 117 before the hijri this and this happened).
As a sidenote, what's quite interesting is that these Muslim writers were pretty well-aware of the calendar customs of other peoples. If I remember correctly, Tabari also gives the Roman dating system, although I'm not sure if it's AUC or based on the Roman incorporation of Nabataea (which is a more common calendar system in pre-islamic inscriptions, albeit dates are rarely given there), and I'm pretty sure Ibn al-Nadim also knew the Greek and Jewish dating systems.