Ronald Brent left Earth on August 6, 2109.
Ronald Brent was inspired by the character of John Brent (James Franciscus) from Beneath the Planet of the Apes (1970).
According to Ron Brent Cape Kennedy was turned into a space museum at least 60 years before he was born, which would be around 2019. After that, space flights were launched from a site in the Mojave desert.
Flames of Doom (1975) established that Ronald Brent was born on May 2, 2079. He left Earth on August 6, 2109, aged 40, and estimates that it has been some 15 to 20 years since he crash landed on the Planet of the Apes, which would make him nearly 60 years old.
This episode originally aired as #9.