Why is it difficult to determine the dates of the Avesta’s different texts?

Determining the precise dates of the Avesta’s various texts is difficult primarily because the collection was transmitted orally for centuries before being recorded in writing.12

The primary challenges include:

• Oral Tradition: Unlike other ancient Middle Eastern religious texts, the Avesta was not reduced to writing during the ancient period. It was composed and transmitted orally by Zoroastrian priests, meaning the texts were not fixed in time or place during their initial development.12
• Late Transcription: The language of the Avesta had ceased to be in common use long before it was finally recorded in a special Avestan script during the 6th century CE. Consequently, the extant manuscripts are relatively recent, dating only from the 13th century and later.12
• Editing and Redaction: The Avesta is a compilation of diverse texts written by different authors over a significant span of time. Throughout its history, the collection underwent multiple stages of editing and redaction, including a major compilation under the Sasanian dynasty in the 6th century CE, which complicates efforts to isolate the original composition dates of individual sections.21