The date of Zarathushtra has always been one of the major problems in Iranian studies, so much so that it has been said, with inappropriate irony, that the study of it has always been the favorite pastime of scholars.
The entire information about the date of Zarathushtra, which has come down to us, is tinted by cosmological speculations, so in order to be able to evaluate correctly the chronological data a discussion of the cosmological ideas and their history is unavoidable.
The religious chronology is more probably due to a speculation which is based upon the old 300-600-900 years calculation reflected in the Yima (Pers. Jam) story of the Avesta, which was revived in the early Sasanian Period and synchronised with the historical events of that time; that is to say, the Avestan chronology was modified in the Sasanian period in order to be adjusted to current religious and political requirements. The chronological indications found in the seventh book of the Dêngird, are due to such a modification.
In Dêngird a prophecy is retold which establishes a speculative chronology. The place of Zarathushtra lies at the turn of the third to the fourth period. This implies that the most imprtant event of his life, be it his revelation at the age of thirty, was considered to have taken place in the year 9001 of the cosmical year. Avesta enumerate the sovereigns (and high-priests) that will come after Vîshtâspa (and Zarathushtra) until the end of the Aryan kingdom (Pers. erân shahr).
First, among the sovereigns Vohmanah, son of Spentodâta, about whom it if said this in the Avesta that: «The tighteous Vohumanah who is the most challenging amongst the Mazdaeans». And among the high-priets Saêna, as about him it is said this: «The Religion becomes a hundred years old when Saêna is born, and two hundred years when he passes away». And this also: «We worship th Fravashi of Saêna (son) of Ahûm.stût who first went out through this earth with one hundred disciples».
Then, when the Religion becomes three hundred years old, will come a devastator, a Mairye (Villain). It is said this in the Avesta that: «I reveal to you (ô Zarathushtra) which sigs will appear when this thy Mazdaean Religion becomes three hundred years [old]; in the day benightedness arises, the material Existence (ahu- astvant-) sees at the time of the apparition of [that] Mairya [on that day] the planets and also the stars …» It is said this that: «[In] that ever-night winter, that Aêshma (Wrath) would create, secretly, among the creaure of Âzi (Greed), a Mairya (Villain)». The Dêngird identifies this Mairya as the evil man who attacked the Persian kingdom in the fourth century B.C., that is Alexander. After this manner, the “traditional” date of Zarathushtra appears. The date 300 years before Alexander’s invasion virtually suggested by the book Dêngird and which is equivalent to 630 B.C., cannot be taken as a historical reality. This date reflects the pious feeling of the Zarathushtrians of the early Sasanian period.
Then, when the Religion becomes six hundred years old, will come a re-organizer of the Aryan Kingdom (erân shahr) and a re-organizer of the Religion (daênâ mazdayasni). The Avestan name of that re-organizer of the Aryan kingdom is Karsna who will build a new empire called Hvâfrita; and the Avestan name of that re-organizer of the Zarathushtrian Religion is Avarathrabah.
About Karsna, it is said this in the Avesta that:
We worship the Fravashi of righteous Karsna (descendant) of Zbaurvant, brave, personifying the (sacred) Word, the Ahuric wielder of a heavy cudgel, in whose house good, beautiful, radiant Ashi used to walk about in the form of a beautiful, very strong, well-built, high girdled, erect maiden, of noble seed, in respect to (her) illustrious lineage, who (i.e., Karsna) being unequalled in battle (was) best at making room for himself with his own arms, who being unequalled in battle (was) best at fighting against the foe with his own arms.
And about Avarethrabah it is said this that:
We worship the Fravashi of righteous Avarethrabah [son] of Râshtare.vaghenti who descended from the family of Manush.chithra". And this also: "The steel age in which will be born that man, Avarethrabah, the organizer of holiness and organizer of righteousness, and challenging.
The Dêngird identifies that reorganizer of the Aryan kingdom (i.e. Karsna) with Ardashêr son of Pâbag; in this manner, the Sasanian dynasty is considered as the Huâfîdân (Av. hvâfrita). At the same time, Avarethrabah is identified with Âdarbâd son of Mahrspend.
During the early Sasanian period the reorganisation of the Zarathushtrian Religion and the restoration of its power was undertaken by Tansar and Âdarbâd, the supreme priests of the Aryan kingdom. The authorities of that time certainly considered this enterprise as a step towards the Renovation of the World.