When did the Muslim calendar start?
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، حَدَّثَنَا عَبْدُ الْعَزِيزِ، عَنْ أَبِيهِ، عَنْ سَهْلِ بْنِ سَعْدٍ، قَالَ مَا عَدُّوا مِنْ مَبْعَثِ النَّبِيِّ صلى الله عليه وسلم وَلاَ مِنْ وَفَاتِهِ، مَا عَدُّوا إِلاَّ مِنْ مَقْدَمِهِ الْمَدِينَةَ.
Narrated Sahl bin Sa`d: The Prophet's companions did not take as a starting date for the Muslim calendar, the day, the Prophet (ﷺ) had been sent as an Apostle or the day of his death, but the day of his arrival at Medina.