First to etablish hijri calendar
Sa’eed ibn al-Musayyib said:
The first one to establish the (hijri) calendar was ‘Umar ibn al-Khattab (رضي الله عنه) two and a half years into his caliphate. He reached a decision in consultation with ‘Ali Ibn Abu Talib (رضي الله عنه).
[Tarikh al Islam by Imam Dhahabi]