<P> Old Persian inscriptions and tablets indicate that early Iranians used a 360 - day calendar based on the solar observation directly and modified for their beliefs . Days were not named . The months had two or three divisions depending on the phase of the moon . Twelve months of 30 days were named for festivals or activities of the pastoral year . A 13th month was added every six years to keep the calendar synchronized with the seasons . </P> <P> The first calendars based on Zoroastrian cosmology appeared in the later Achaemenid period (650 to 330 BCE). They evolved over the centuries, but month names changed little until now . </P> <P> The unified Achaemenid Empire required a distinctive Iranian calendar, and one was devised in Egyptian tradition, with 12 months of 30 days, each dedicated to a yazata (Eyzad), and four divisions resembling the Semitic week . Four days per month were dedicated to Ahura Mazda and seven were named after the six Amesha Spentas . Thirteen days were named after Fire, Water, Sun, Moon, Tiri and Geush Urvan (the soul of all animals), Mithra, Sraosha (Soroush, yazata of prayer), Rashnu (the Judge), Fravashi, Bahram (yazata of victory), Raman (Ramesh meaning peace), and Vata, the divinity of the wind . Three were dedicated to the female divinities, Daena (yazata of religion and personified conscious), Ashi (yazata of fortune) and Arshtat (justice). The remaining four were dedicated to Asman (lord of sky or Heaven), Zam (earth), Manthra Spenta (the Bounteous Sacred Word) and Anaghra Raocha (the' Endless Light' of paradise). </P> <P> The Parthians (Arsacid dynasty) adopted the same calendar system with minor modifications, and dated their era from 248 BCE, the date they succeeded the Seleucids . Their names for the months and days are Parthian equivalents of the Avestan ones used previously, differing slightly from the Middle Persian names used by the Sassanians . For example, in Achaemenid times the modern Persian month' Day' was called Dadvah (Creator), in Parthian it was Datush and the Sassanians named it Dadv / Dai (Dadar in Pahlavi). </P>

When did we start using a 365 day calendar