Aristotle reckoned the date of the first Olympics to be 776 BC, a date largely accepted by most, though not all, subsequent ancient historians. It is still the traditionally given date and archaeological finds confirm, approximately, the Olympics starting at or soon after this time.
Olympiad calendar
The historian Ephorus, who lived in the fourth century BC, is one potential candidate for establishing the use of Olympiads to count years, although credit for codifying this particular epoch usually falls to Hippias of Elis, to Eratosthenes, or even to Timaeus, whom Eratosthenes may have imitated. The Olympic Games were held at four-year intervals, and later, the ancient historians’ method of counting the years even referred to these games, using Olympiad for the period between two games. Previously, the local dating systems of the Greek states were used (they continued to be used by everyone except the historians), which led to confusion when trying to determine dates. For example, Diodorus states that there was a solar eclipse in the third year of the 113th Olympiad, which must be the eclipse of 316 BC. This gives a date of (mid-summer) 765 BC for the first year of the first Olympiad. Nevertheless, there is disagreement among scholars as to when the games began.
he only competition held at first, according to the later Greek traveller Pausanias who wrote in AD 175, was the stadion, a race over about 190 metres (620 feet), measured after the feet of Hercules. The word stadium is derived from this event.
Source: Wikipedia
