Dates appear regularly on the tetradrachms from the reign of Phraates IV onwards. Before this time they occur rarely and sporadically. Some of the later bronze coins--having rather the character of civic issues--bear dates, but the drachms are never dated.
The era employed is the Seleucid, dating from the autumn of B.C. 312.
The dated tetradrachms usually record the name of the month as well as the year, the moths being those of the Macedonian Calendar, which had become known in the East through Alexander's conquests. The year consisted of twelve, sometimes of thirteen months (i.e. moons). The place in the year assigned to the intercalary month (EMBOLIMOV) is not known, but the most convenient hypothesis seems to be to assume that it was at the end of the year.
The months are:
1. DIOS (= October)
2. APELLAIOS (generally written with only one L)
3. AUDUNAIOS (sometimes written AUDANAI)
4. PERITIOS (generally written PERITEI)
6. XAUDIKOS (sometimes written CANDIKOS, on coins of Pacorus II as XANQIK)
10. LWOS (frequently written as OLWIOU, OLWOU, LOIUS, LOUS)
The first month probably began with the autumnal equinox, which is near October 1st, and therefore in the majority of years the greater part of Dius fell in October.
Wroth, Warwick, BMC Parthia