Although the Parthians were speakers of an Iranian language, administrative and official documents from the time of their reign (the Arsacid period) are in the vast majority in other languages, e.g. in Greek and Babylonian, and accordingly use the Macedonian and Babylonian calendars. A certain number of texts in the Parthian language are also preserved. Calendrical terms found in these documents are taken from the so-called ‘Zoroastrian calendar’. Many varieties of this calendar, which names the months and days after deities of the Zoroastrian religion, have been used throughout the millenia and in various Iranian languages, ranging from Old Persian of the Achaemenid period to the Persian calendar of present-day Iran. The systems according to which dates of the Zoroastrian calendar are calculated has varied considerably, depending on the cultural context. For instance, the Zoroastrian calendar patterned on the Babylonian lunisolar year in the Achaemenid period at least for administrative purposes, while in Sasanian times, it referred to a year of 365 days. The origin of the ‘Zoroastrian calendar’ and its use in Sasanian and later times (involving a number of complex calendar reforms) has received a considerable amount of attention while it is not quite clear yet how it was used in the « ill-documented Seleucid and Parthian periods » (Boyce 1983, 792). The aim of this paper is to investigate the use of the Zoroastrian calendar in Arsacid times, and to try to fi nd out whether it patterned on one of the other calendars used at that time, or followed a diff erent system. It will specifi cally discuss the hypothesis that, at least for administrative purposes, the Parthian Zoroastrian calendar might have patterned on the Macedonian one, which was used by the same administration for the same purposes, including dates on the Arsacid coins. The discussion will limit itself to a discussion of the relevant Parthian texts, and to exploring which conclusions they permit, or do not permit, about the calendar involved.