An Olympiad is the period of four years from one Olympic game until the next, starting in 776 B.C. The last Olympic game, the 283d, ended in A.D. 394 when Emperor Theodosius I banned the pagan games. Olympiads began with the games that started with the full moon closest to the summer solstice. Dating them was modified with the introduction of the Metonic Calendar. The Antikythera Mechanism
from around the second century B.C. is thought to have kept track of Olympiads.