An Olympiad (Greek: Ὀλυμπιάς, Olympiás) is a period of four years associated with the Olympic Games of the Ancient Greeks. Although the Ancient Olympic Games were established during Archaic Greece, it was not until the Hellenistic period, beginning with Ephorus, that the Olympiad was used as a calendar epoch. Converting to the modern BC/AD dating system, the first Olympiad began in the summer of 776 BC and lasted until the summer of 772 BC, when the second Olympiad would begin with the commencement of the next games. Thus, Olympiad N for N less than 195 started in the year
780
−
4
×
N
{\displaystyle 780-4\times N}
BC and ended four years later. For N greater than 194, Olympiad N started in AD
4
×
N
−
779
{\displaystyle 4\times N-779}
and ended four years later.
By extrapolation, the
1st year of the 700th Olympiad began (or will begin) around June 21, 2021.
A modern Olympiad refers to a four-year period beginning January 1 of the year the Olympic Summer Games are normally held. The first modern Olympiad began January 1, 1896, the second January 1, 1900, and so on (the 32nd began January 1, 2020: see the Olympic Charter).
The ancient and modern Olympiads would have synchronised had there been a year zero between the Olympiad of 4 BC and the one of 4 AD. But as the Julian calendar goes directly from 1 BC to 1 AD, the ancient Olympic cycle now lags the modern cycle by one year.