Julian day
The first Hijri year (AH 1) was retrospectively considered to have begun on the Julian calendar date 15 July 622 (known as the ‘astronomical’ or ‘Thursday’ epoch, Julian day 1,948,439) or 16 July 622 (the ‘civil’ or ‘Friday’ epoch, Julian day 1,948,440), denoted as "1 Muharram, AH 1". Years prior to this are reckoned in English as BH ("Before the Hijra").