Every nation had his own calendar. Usually people counted years by kings (in the nth year of king X’s reign), this is called a regnal year. But ancient historians sometimes used a (often mythical) date of origin of their city.

Example : Romans usually used the consul of the year in everyday life (“in the year when Cicero was consul”) but for history work they counted “AUC”/ad urbe condita, meaning from the foundation of the city, the mythical date of the foundation of Rome of 753BC. We only stopped using that date as year zero one around the 6th century btw.

Other examples: the ancient Greeks counted from 776 thought to be the first Olympic Game (a very important cultural Panhellenic event).

The regnal count was still the most common. Still used today: in Japan date (for ceremonial purposes) is sometimes recorded as year X of an era (“era” being here the rule of an emperor, 2022 is the fourth year of the Reiwa era for example).