How was the 12th of Rabi' al-Awwal determined as the birthday of the Prophet (PBUH), even though the Hijri calendar began after his birth?

The Hijri date was not used during the Prophet's era, but the Arab months were known. Therefore, if it is possible to know the date of a day from any month, there is nothing strange about it. Scholars did not date his birth, peace and blessings be upon him, by the Hijra, but rather said he was born in the Year of the Elephant. It is well-known among people that he was born on the twelfth of Rabi' al-Awwal, and this was transmitted by Al-Dhahabi, Al-Tabari, Al-Bayhaqi, and others. Ibn Sa'd mentioned that he was born on the second, and some said on the eighth day, which was preferred by more than one.