If you have a hijri (lunar) year:

common era year = Hijri + 622 - (Hijri/33.18)

So, if you have year 656 and you want to know the common era year: 656+622-(656/33.18)=1258.23 = 1258

If you have a common era (solar) year:

Hijri = (CE-622) / .9698614

So, if you have 1258 and you want to know the hijri year: (1258-622)/.9698614= 655.76 = 656