Could you please clarify the different dates for the birth of the Messenger, may Allah bless him and grant him peace? Did the difference arise only from the adoption of the Hijri calendar? And is the 12th of Rabi' al-Awwal the fixed date?

Historians and biographers differ on the exact birth date of the Prophet Muhammad, peace and blessings be upon him. However, it is agreed that he was born on a Monday in the month of Rabi' al-Awwal, in the Year of the Elephant, corresponding to the twentieth or twenty-second of April in the 571st year of the Gregorian calendar. As for the specific day in Rabi' al-Awwal—whether it was the second, eighth, ninth, or twelfth—there is disagreement. Most biographers lean towards the twelfth, while the scholar Mahmoud Pasha concluded that it was on the ninth of Rabi' al-Awwal.