The Bostran Era (also called Arabian Era) started corresponding to March 22, 106 AD. This was the official dating of the Roman Province Arabia Petraea, and it was introduced in order to make it coincide with the regnal years after the inclusion of the Nabatean Kingdom to the Roman Empire. It has the name of the city of Bostra, that soon was the home of the Sixth Legion.
The start date of the Bostran Era was a controversial matter, in part because the Chronicon Paschale states that it started at the time of the consuls Candidus and Quadratus (105), while the discoveries of the Cave of Letters´ manuscripts made it clear that the Bostran Era started on 106.
The Bostran year was lunisolar: it has 12 months with 30 days with five epagomenal days at the end of each year. The names of the months came from the ancient Macedonian calendar, although Nabatean names were also introduced. From the second Bostran year on, a leap year was introduced every four years, having a sixth epagomenal day. The first day of the first month (Xanthikos) of the year therefore was equivalent to March 22 in the Julian Calendar, matching with the spring equinox.
This calendar was used in Jewish-Palestinian texts, as well as Nabatean from the Aramaic, in Greek and in Arabic. The inscriptions from Arabia Petraea, in which only a number is written, this number often corresponds to the year of the Bostran Era. In any case, in spite of the denomination “Bostra”, this province was not the most important, being Petra much more important in the first years. In fact, the use of the Bostran Era has no special connection with the city, other than the fact that it was the military Roman base, as a symbol of the annexation of Nabatea as a province of the Roman Empire.
The Bostran era dating was used in commemorative and honorific inscriptions, and in administrative and legal documents. As it was not mandatory to use this new calendar, many cities continued to use local calendars on locally minted coinage. These included the Pompeian era (63 BC) in some cities of the Decapolis and the era of Capitolias (AD 97/98). The Bostran era was probably a local response to the political changes which made the old Nabataean regnal year numbering impossible.
The oldest surviving examples are found in a Nabataean inscription at Oboda from AD 107, and a Nabataean papyrus from Naḥal Ḥever (AD 120). The earliest example in Greek is from a papyrus also from Naḥal Ḥever (AD 125). There´s also an official inscription of the Emperor Gordian III at Bostra (AD 238/9) using the provincial era.
There is some uncertainty whether the era of Arabia was ever used outside the province of Arabia while the Roman administration was still intact.
The use of the era spread with the province of Arabia and its successors. There are several Christian inscriptions of the late fifth and early sixth centuries in the Arabic script that bear dates in the Arabian era. The use of the provincial era continued well into the Islamic period, even as late as AD 735, although in the later period, the calendar era was almost never identified explicitly.