This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
As in the Islamic calendar (section 4.3), years are counted since
Mohammed's emigration to Medina in AD 622. At vernal equinox of that
year, AP 1 started (AP = Anno Persico/Anno Persarum = Persian year).
Note that contrary to the Islamic calendar, the Persian calendar
counts solar years. In the year AD 2003 we have therefore witnessed
the start of Persian year 1382, but the start of Islamic year 1424.