The Magi not only revitalized the legendary history of the millennium of the religion and reinterpreted it with reference to the actual history of Persia, but also equated the beginning of the millennium to the historical era(s).
The Old Persian Era used by the Magi was from the year 330 B.C. in which Dārayavahu (Darius III) was killed. After the fall of the Sasanian kingdom, the Magi anew used to date from the year in which Yazdegird was killed (that is, 652 A.D.). This era was called pārsīg ‘Persian’ (sāl ī pārsīg) , or المجوس ‘Magian’ (تاريخ المجوس). However, the Persian scribes used the Æra Yazdegirdi in the astronomical tables, the epoch of which is the beginning of his reign. The early Sasanian scholars were familiar with the Seleucid era called by the Parthians and Persians Former Reckoning (πεότερος), known also as Æra Alexandri (312-311 B.C.) and or anno astronomorum Babyloniae (311-310 B.C.), and the Arsacid era called Royal Reckoning (βασίλειος), identical with the former era except for beginning of the epochal year 64 years later. In the early Sasanian period, the initial dates of these three eras were identified with the first year of the tenth (and or twelfth) millennium of the world year.
Anno religionis
We find three kinds of interpreting the opening year of the tenth millennium (Capricornus):
1. The era of Zaraθuštra is the old Persian “era” with the epoch date -329 April (Daθuš 1).
This results in the shortening of the first period of the religion to 258 years and the curtailment of the duration of the Arsacid period to 284 years.
Example:
According to Bērōnī in the Canon Masudicus, Zaraθuštra was 276 years before Æra Alexandri. The death of Dārayavahu was in July (or August) of the year 330 B.C. The Seleucid era was reckoned from the Babylonian New Year, and began on 3 April 311 B.C. (i.e., six month later than the Macedonian Seleucid era with the epoch date 312 B.C., Oct. 1). Thus Æra Alexandri started 18 years after Alexander’s conquest of Persia (and or the death of the last Achaemenian king).
258 + 18 = 276.
2. The Royal era was taken as the era of Zaraθuštra.
The difference of 82 years between the initial dates results in the re-shortening of the Arsacid period to 202 years.
Examples:
One. From the Bundahišn, 240
aškānān <ī> ped ahlav-xvadāyīh nām barend, 200 ud and sāl.
The Arsacids who bear the name of the “Truthful Rule” (OPers. arta-xšaça-) ruled something over two-hundred years.
Two. The Šāhnāma (possibly based on the official Persian book of the kings, the Xvadāy-nāmag) assigned two hundred and odd years to the Arsacid kings: «Thus elapsed some two hundred years » (M 21, 49) between the reign of Arsaces I (Aršaka) in 247 B.C. and the accession of Ardašēr.
3. The beginning of the Former (Seleucid) era was referred to, by some Persian scribes, as “the coming of the religion” or “the appearance of Zaraθuštra”.
The 538 years from the beginning of the Seleucid era to the accession of Ardašēr were divided into three periods: 258 from “the appearance of Zaraθuštra” to Alexander’s conquest of Persia (and or the death of the last Achaemenian king); 14 years for Alexander’s reign; 266 years covering the rule of the Arsacids.
Bērōnī reported that: The Persians and Magians count 258 years from the appearance of Zaraθuštra till the beginning of the Æra Alexandri (Vestiges, 3,3.). Bērōnī himself counted 1221 years from his appearance till the death of Yazdegird III.15 This means that Bērōnī and some Persians placed the beginning of the millennium of the religion in 570 B.C., i.e. twelve years before the supposed conversion of Vīštāspa in 558 B.C.
Example. The story of the Cypress of Kašmar.
There is a story, told by Daqīqī in the Šāhnāma, and narrated in a number of Arabic and Persian books, about the planting of a cypress-tree (by the order of Vīštāspa) by Zaraθuštra who had brought from Airyana Vaējah, at the gate of the fire-temple at Kašmar, in the district of Taršīz in Xvarāsān, as a memento of Vīštāspa’s conversion, and had inscribed upon the noble cypress that: “Vīštāspa accepted the Good Religion”.
The Khalif Mutawakkal ordered in the year 232 of the Hejira (A.D. 846 Aug. 28- A.D. 847 Aug. 16) the cypress to be cut off. It had stood for 1405 years from the time of its being planted, to the date of its cutting down by the Muslims.19 If we take solar (calculated) years, we arrive at 558 B.C. as the date when the tree was planted.