Most Iranists are probably acquainted with some of my writings on Iranian chronology and systems of time-reckoning. One of the essential points in my outline of the history of the Iranian calendars is the theory I put forward about the date of the creation of a fixed religious year, co-existing and running parallel with the civil year, which was a vague one. This fixed year, called in Pahlavi literature Vihēǰakīk , was, however, theoretical and without any application in daily life. It was used only by the Zoroastrian clergy for the purpose of ascertaining the original position of the religious festivals and keeping them in or about their astronomical positions. By the ‘original position’ is meant the places these most important religious days occupied in the tropical year at the time of instituting the Vihēǰakīk year, which time may or may not have been the epoch of the initial adoption of the Mazdayasnian or ‘Young Avestan’ civil year in Iran. The date I proposed was 441 B.c. , or at any rate some time in the first decade of the second half of the 5th century B.c.
S. H. Taqizadeh (1952) studied this question.