What is the legitimacy of the Hijri dating system in light of the Qur'an and Sunnah?

The Companions, during the time of Umar ibn Al-Khattab, agreed to date by the Hijrah of the Prophet, starting from Muharram, due to its conformity with the Prophet's saying (peace be upon him): "Hold firmly to my Sunnah and the Sunnah of the Rightly Guided Caliphs after me." Allah has placed truth on Umar's tongue and in his heart. This action has been agreed upon by the Ummah, both predecessors and successors, and opposing the path of the believers is forbidden, as stated in His Almighty saying: "And whoever opposes the Messenger after guidance has become clear to him and follows other than the way of the believers - We will give him what he has taken and drive him into Hell, and what an evil destination!" As Ibn Masud said: "What the Muslims deem good, is good in the sight of Allah."