A fundamental characteristic of the Islamic calendar is that it marks the establishment of the Islamic era. The importance of the Islamic calendar was inculcated in the Prophet’s ﷺ companions, who strongly adhered to the order and number of months, and would implement it in a more formal manner after the passing of the Prophet ﷺ. As the Islamic empire grew, legal rulings, deeds, and other important documents depended heavily on dates as a means of communication and preserving order. While the months and days of the calendar were well established, the companions still faced challenges in chronicling events on a yearly basis as it was unknown which year they were referring to when deeds and documents arrived with no complete date. In his historical chronicle, Tārīkh al-Rusūl wa-al-Mulūk, Ṭabarī (R) states:

Maymūn b. Mihrān (R) narrates: A legal document for a deed was delivered to Hadhrat Umar (RA) which had the month of Shaʿbaan written on it. Hadrat Umar (RA) asked: Is this the Shaʿbaan of last year or this coming year? Then he said to the companions: Let us determine an epoch (unit of time) for the people to use.

Throughout human history, people have used memorable events as reference points for establishing epochs.
This system continued into the caliphate of the Second Caliph Umar b. al-Khaṭṭāb (RA) until the companions came to a consensus on using the migration of the Prophet ﷺ as the starting point of the Islamic era. Ṭabarī mentions a few narrations that eventually led to the advent of the Islamic era:

Abū Mūsá ʿAbd Allāh b. Qays al-Ashʿarī (RA) wrote to Hadhrat Umar (RA): Letters have come to us which have no date. ʿUmar (RA) then gathered his advisors. Some of them suggested using the revelation to the Prophet ﷺ as [the starting point of] the era. Others suggested basing it on the Prophet’s ﷺ migration. ʿUmar (RA) then replied: We will base it on the migration of the Prophet ﷺ, for the migration is what separated truth from falsehood.

We know with certainty that these discussions occurred during the caliphate of ʿUmar (RA). We also know that choosing the migration as the starting point of the Islamic era was a matter of informed judgment (ijtihād), and that the companions came to a consensus on the matter. Thus, following the Islamic calendar involves not only obeying Allah ﷺ and His Messenger ﷺ, but also relying on the consensus of the companions in preserving Islamic identity.

In brief, the Islamic calendar is not merely a twelve-month lunar calendar with four sacred months, but also a marker of Islamic identity by virtue of the institutionalization of the first Muslim community 1442 years ago.