At present, there are three different Zoroastrian calendar-traditions; Seasonal or Avestan, Old Reckoning or Qadimi and the Shenshái or Royal. Parsis of India follow mostly the royal or Shenshái calendar. The old reckoning and royal zoroastrian calendars start each of the 30 day long months with the Sun entering a new constellation, similar to the Vedic (Hindu) Solar calendars as reflected in the Jyótishá (Vedic Astrology), and the Armenian calendar, but different from the Seasonal Avestan Calendar, whose epochs of the months are fixed to the equinoxes/solstices.
All three different Zoroastrian calendars consist of 12 months or Máhs, and each month has 30 days. Each of these days is known as a Róž, and each Róž is named after one of the names, virtues, powers of Ahúrá Ma(n)zdá. There are 5 gáthá or enchanting days after the 12 months, in ALL the calendars.
The difference is that the old reckoning/qadimi and royal/shenshái calendars do NOT add a sixth day in leap years, as is the case with the Seasonal Avestan calendar. A solar calendar is around 365 ¼ days, which the Seasonal Avestan calendar accommodates by adding a day every four years (a leap day called avardaad,) or an extra month every 120 years as prescribed in Denkart (III.419.) Because of this difference, the qadimi and shenshái calendars and Avestan solar year began to diverge.
Thus the qadimi and shenshái variant of the Zoroastrian calendar roam through the seasons, while the Avestan does NOT. Furthermore, both the qadimi and shenshái variant of the Zoroastrian calendar use the Y.Z. suffix (Yazdgerdi Era) for their calendar era (year numbering system), indicating the number of years since the coronation in 632 CE of Yazdgerd III, the last emperor of the Sássánian dynasty. This tradition is similar to the Japanese tradition of reckoning the years from the coronation of their emperor. i should add that the local claendar of the inhabitants of the Caspian Mountains in Iran, bears an almost identical similarity to the qadimi and shenshái variant of the Zoroastrian calendar.
Now what about the difference between the qadimi and shenshái calendars???? In 1006 CE, the roaming New Year’s Day once again coincided with the “Avestan nauvrooz” or day of the vernal equinox. According to legend it was resolved that the royal calendar henceforth add an additional month every 120 years. The Zoroastrians in India hence added an extra month to the calendar around 1129 CE. The parsis last remembered to add this extra month in 1129 CE. Consequently, New Year, which originally correlated with the vernal equinox on March 21, has since fallen earlier in August. Meanwhile, the Zoroastrians who remained in Iran NEVER ONCE added a thirteenth month.
Around 1720 CE, an Irani-Zoroastrian priest named Jámásp Peshótan Veláyati traveled from Iran to India. Upon his arrival, he discovered that there was a difference of a month between the Parsi calendar and his own calendar. Dastoor Veláyati brought this discrepancy to the attention of the priests of Surat in india, but no consensus as to which calendar was correct was reached. The question was posed to the learned iranian priesthood; they ruled that parsis have become a distinct group, hence are free to keep their calendar and traditions.
In 1736 a layman by the name Jamshid, well-versed in astronomy, came to Surat, where he stayed for three years. He befriended Dastoor Kaús Fardún-ji Monajjem, and taught him astronomy, thereby convincing him of the need to adjust the Parsi royal calendar. Monajjem became the first qadimi dastúr. Around 1740 CE, some additional parsi priests argued that since their visitor had been from the ancient ‘homeland’, his version of the calendar must be correct, and their own must be wrong.
Mulla Feróz (d. 1830), the son of dastoor Kaús, succeeded his father as the dastúr of the Dadyseth Ātaš Bahrām, and in 1826 Mulla Feróz played a leading role in resolving a quarrel between shenshais and qadimis. In Bombay the qadimis were influential, as is documented by their foundation of the first and the third of the four Ātaš Bahrāms. Members of qadimi families, such as Dadyseth, Banaji Cama, and Vatcha, included famous traders and great philanthropists, such as the Zoroastrian scholar and community leader Khúrsheed-ji Rústam-jee Cámá (1831-1909).
In 1906, Khúrsheed-ji Cámá, founded the “Zarthóshti Fassli or Zoroastrian Seasonal-Year Society. The Fassli calendar as it became known, was based on the ancient Avestan model. It was originally reintroduced in 1079 by “Omar Khayaam,” during the reign of a Seljuk ruler and had been well received in agrarian communities.
The calendar had two salient points: 1) It was in harmony with the seasons and New Year’s Day coincided with vernal equinox. 2) It followed the Avestan model exactly (12 months of 30 days each plus 5 extra days), had an auto-regulatory leap day every four years: the leap day, called Avardaad-caal-, followed the five existing Gatha days at the end of the year.
The Seasonal society claimed accurately that their calendar was an accurate religious calendar, as opposed to the other two calendars, which they correctly asserted were only cultural/historical.
The new calendar received little support from the Indian Zoroastrian community. In Iran, however, the seasonal calendar based on the ancient avestan model, gained momentum. In 1925 the Iranian Zoroastrians with the rest of the Iranian Nation adopted the new calendar of the seasons. The adopted seasonal calendar had also retained the ancient Zoroastrian names of the months.
The seasonal calendar was duly accepted by the great majority of the Iranian Zoroastrians. In rural villages of Yazd, however, the Zoroastrian villagers resisted, and to this day follow the old reckoning or qadimi calendar.