text: The Parthians (Arsacid dynasty) adopted the same calendar system with minor modifications, and dated their era from 248 BC, the date they succeeded the Seleucids. Their names for the months and days are Parthian equivalents of the Avestan ones used previously, differing slightly from the Middle Persian names used by the Sassanians. For example, in Achaemenid times the modern Persian month 'Day' was called Dadvah (Creator), in Parthian it was Datush and the Sassanians named it Dadv/Dai (Dadar in Pahlavi).
  When in April of AD 224 the Parthian dynasty fell and was replaced by the Sasanid, the new king, Ardashir I, abolished the official Babylonian calendar and replaced it with the Zoroastrian.   This involved a correction to the places of the gahanbar, which had slipped back in the seasons since they were fixed.   These were placed eight months later, as were the epagomenai, the 'Gatha' or 'Gah' days after the ancient Zoroastrian hymns of the same name.   Other countries, such as the Armenians and Choresmians, did not accept the change.   The new dates were:
  In AD 224 the vernal equinox at Greenwich fell at noon on 21 March, which was 22 Shahrewar.   Immediately after the reform 21 March corresponded to 27 Shahrewar.   Here is the calendar for AD 225–6: