If I was alive in 500 BC and asked someone what year it was, what would they have said?

In Rome, they would tell you it was the year of the consulship of Camerinus and Longus, the two consuls of the time. If the Romans were specifically recording a length of time, they might also tell you it was 253 years "ab urbe condita" - "since the founding of the City". They might also say it was in the ninth year of their Republic.

they are using what is called regnal year or the year based on a sovereign king ruling. The tradition of measuring years about the reigns kings really helped standardize the years in Europe when the 'King of Kings' calendar began. so the calendar that we are using now are based on this concept of kings.