Pahlavi writings tell us that the religious era began from the day Zarathushtra proclaimed his Divine Mission to humanity.  This era, based on the astronomical calculations that Zarathushtra declared his mission on the vernal equinox when, according to the precession, the period of Aries is supposed to have begun, comes to be 3738/39 in 2001 CE i.e. 1737 BCE. It has been called the "Year of Religion" in Pahlavi writings. The Zarathushtrian Assembly calls it the Zarathushtrian Religious Era (Z.E.R./ZRE) and has, since its establishment in 1990, observed it as the beginning of the Zarathushtrian calendar.  The Zartoshti community in Iran joined in to observe ZRE as its calendar in 1993, and many Irani Zartoshtis in diaspora have also accepted it.  Earlier, each of the Iranian kings, following the pattern set by other Middle Eastern rulers, particularly the Babylonians, observed a new era from his own ascension to the throne.  With as many as 80 rulers on the Iranian throne during the thousand and odd years of Achaemenians, Macedonians, Parthians, and Sassanians, much confusion in chronology has arisen, and many dates have been misinformed, misused, misplaced, misinterpreted, miscalculated, and missed.  The Yazdgerdi era reminds one of the last Emperor who got overthrown by Arab invaders.  It is not a happy recollection.  Sassanians and Two Calendars: The Sassanians continued to maintain both the "yaairi" of 365 days and the "saredha" of 365.24224 days.  The first they called "oshmurdik" meaning "rememberable, reckonable" and the second "vihezakik" meaning "moving, progressive, intercalary."  While the "rememberable" was easy for the laity to memorize and count them by names, the "intercalary" belonged to the astronomer priests, linked with the imperial court, to keep the formal year precise and in tune with the seasons.  The fall of the Sassanian Empire fell the astronomer priests of their high position.  Nevertheless, the intercalary year was, Pahlavi books and the present position of the Qadimi and Shahenshahi calendars tell, kept until the 11th century CE.  The decline of astronomer priests put an end to Vihezakik and the lay priests have continued with their "Ushmordik," advancing about one day in every four years out of the season and the solar year.  Economic and seasonal revenue collection, however, forced the Muslim Caliphs to maintain, evidently by those astronomer priests who had embraced Islam, the intercalary year in addition to the Islamic calendar of a purely lunar year.  It was this Vihezakik year maintained halfheartedly by Muslim rulers, which was improved, perfected and formally restored by Omar Khayyam and other Iranian scientists.  It was named the "Jalaali" calendar after its patron, Sultan Jalal al-Din Malekshah Saljuqi (1072-1092 CE).  The Fasli year, officially observed by Iranians -- Zartoshtis, Jews, Christians, and Muslims -- in modern Iran, is the "saredha" of the Avestan people, "tharda" of the Achaemenian, "Vihezakik" of the Sassanians, and the "Jalali" of Omar Khayyam.  The precise solar year also reckoned by all observatories in the world. It is the Universal Astronomical and Scientific Year.  It is this Vihezakik (Persian "Behizaki") calendar, now called "Khorshidi" (solar), the official Iranian calendar, the precise calendar, with its dates numbered, that the Zarathushtrian Assembly follows.  It is astronomically precise.  It is progressively Zarathushtrian.