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.  

مَا عَدُّوا مِنْ مَبْعَثِ النَّبِيِّ ﷺ وَلاَ مِنْ وَفَاتِهِ مَا عَدُّوا إِلاَّ مِنْ مَقْدَمِهِ الْمَدِينَةَ  

Originally, two rakʿat were prescribed in every prayer. When the Prophet ﷺ migrated (to Medina) four rakʿat were enjoined, while the journey prayer remained unchanged(i.e. two rakʿat).  

فُرِضَتِ الصَّلاَةُ رَكْعَتَيْنِ ثُمَّ هَاجَرَ النَّبِيُّ ﷺ فَفُرِضَتْ أَرْبَعًا وَتُرِكَتْ صَلاَةُ السَّفَرِ عَلَى الأُولَى