The calendar keeps track of months and years. There is no record of calendars and the way people calculated dates in the pre-Achaemenian era. After the Achaemenids, however, two kinds of calendar were created.
The first calendar was found in Persepolis inscriptions. It consisted of twelve months, probably beginning in autumn. This calendar was a solar calendar, including leap years.
The second calendar was the Avesta calendar which was the origin of the current Iranian calendar.
In ancient Iran lunar months were used in a different way. The week, which was one of the bases of the Semitic calendar, did not exist. Instead, the month was divided into thirty days, each month having a specific name.
The year in the Avestaian calendar was comprised of 365 days which made up twelve 30-day months. The five remaining days were called “Panjeh”.
In the old Persia, the time of the king’s coronation was considered the beginning of the calendar and the years were named after the kings. For example, they said, ‘the fifth month of Ardeshir’s seventh year of rule’. In 247 B.C., beginning with the Parthian era, the origin of the calendar was changed. Beginning with the Sassanid dynasty, again the calendar was changed to that used in the Achaemenian era.
At the time of Yazdgerd, the last Sassanid king, the year 631 A.D. was chosen as a new beginning for the Iranian calendar. Since no king ascended the throne after him, that calendar remained in use as the Yazdgerdi calendar.
In the Sassanid era, collecting taxes by the government started at Nowruz (the first day of the new year). After the Arab invasion of Iran, when Persians were converted to Islam, the tradition of collecting taxes and many other Persian traditions were adopted by the Abbasid caliphs. However, since they did not take the leap year into account, each year the time of Nowruz changed. They then decided to take the leap year into account, as the Persians had nit he pre-Islamic era. Thus originated the Motavakkeli calendar and Mo’tazedi history.

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