The main story of Assassin's Creed Origins begins in 49 BCE.
The game follows Bayek of Siwa in Ptolemaic Egypt during the period when Julius Caesar arrives in Egypt and becomes involved in the struggle between Cleopatra and Ptolemy XIII. The opening events occur in 49 BCE, before the creation of the Roman Empire and before Cleopatra's later alliance with Mark Antony.
Dates in this period are often confused because ancient historians describe overlapping civil wars and because the Roman calendar changed over time. Assassin's Creed Origins also includes story material set later than its opening, including events connected to the formation of the Hidden Ones. The game was designed as a prequel to the later Assassin Brotherhood, so its historical setting predates the first game by more than a millennium.