Origin of calendar dating lies and dating in the city
The first calendars based on Zoroastrian cosmology appeared during the later Achaemenian period (650 to 330 BCE) and though they have evolved and changed over the centuries, the names of the months have remained more or less the same till now.Before this period, old Persian inscriptions and tablets indicate that early Iranians used a 360-day calendar based on the Babylonian system modified according to their own beliefs, and their own named days.Although they followed the new trend of dating by eras, they established their own era of Zoroaster.This marked the first serious attempt to determine the dates associated with the prophet Zoroaster's life.
This incorrect date is still mentioned in many current encyclopedias as Zoroaster’s birth date.
The calendar had a significant impact on religious observations.
Not only did it fix the pantheon of major divinities, but ensured that their names were continually uttered, since at every Zoroastrian act of worship the yazatas of both day and month were invoked.
Their era became known as that of Alexander, or later the Seleucid era.
The Zoroastrian priests, having lost their function at the royal courts since the new rulers were not Zoroastrians, resented the Seleucids.
A thirteenth month every six years was added to keep the 360-day calendar in harmony with the seasons.