So I'm reading DAI (which was written in the 10th century) for a uni essay and in it Constantine says "From their migration from Salona to Ragusa, it is 500 years till this day, which is the 7th indiction, the year 6457."
Where does Constantine get the year 6457 from?
Constantine is counting the years back from the time of the creation of earth. The Byzantines believed this creation to have happened 5509 years before the birth of Christ. This means the year Constantine is talking about here would be 948 AD in our calendar.
The Byzantines had arrived at this date for the earth's creation through studying the chronological and genealogical information provided by the Septuagint, the Greek version of the Old Testament. From the 7th century onwards it seems to have been regularly used as the starting point for the Byzantine calendar.