What is the ruling on my rejection of my brother, who has completed ten years of age according to the Hijri calendar, only to consider his age by the Hijri calendar and insist on calculating it by the Gregorian calendar? And is it correct that the calculation should only be by the Hijri calendar?

The Lawgiver linked all religious obligations, such as fasting, zakat, and Hajj, to the sighting of the crescent moons. The reckoning considered in Islam is the Hijri year, based on lunar months. Allah Almighty says: "Indeed, the number of months with Allah is twelve months in the Register of Allah from the day He created the heavens and the earth; of these, four are sacred" (Surat At-Tawbah: 36). And He says: "They ask you, [O Muhammad], about the new moons. Say, 'They are measurements of time for the people and for Hajj'" (Surat Al-Baqarah: 189). This indicates the obligation to tie rulings to the months and years known to the Arabs.