In the 1968 film Planet of the Apes ANSA astronaut George Taylor and his crew aboard their ship crash-land back on the planet Earth in 3978 Earth Time. This date is found in two different references. The year 3978 is visibly shown on the ship's date-meter moments after crashing into the lake, and it is also found as part of the interactive menu screen on the Planet of the Apes DVD from the Evolution Collection.
However, in the 1970 film Beneath the Planet of the Apes, another astronaut named John Brent lands on Earth some months after the events chronicled in the first film. When speaking with his dying comrade, he indicates that the year is 3955 – more than twenty years prior to the date in Planet of the Apes, conflicting with the pre-established chronology of events.
In the third movie, Escape from the Planet of the Apes, this year is again referenced. While under the influence of Grape Juice Plus, Dr. Zira confesses that the date meter on the ship as the Earth was destroyed read, "...thirty-nine fifty... something". It is important to note that Zira had returned to Earth's past aboard Taylor's, rather than Brent's ship, even though the Liberty 1's terminal never indicated a year of 3955 on-screen. It is also possible that one or both of the ships chronometers were damaged in the crash.
By the fifth movie, when Caesar visits the archives to hear recordings of his parents, we find that the date that Zira states is somewhat garbled, and so Caesar wrongly assumes '3950' as the year for the arrival of Taylor's and Brent's flights and the destruction of the Earth.
The most obvious explanation for this discrepancy is that either the set-designers or writer Paul Dehn simply didn't check the continuity before production. Closer inspection reveals that in his initial script for Beneath, Dehn stated that Taylor was older "by two thousand years and ten" than when he began (Taylor's on-screen date of departure was 1972, though the year 3978 would be 2,010 years after the films release), but Brent in fact gives the current year as "three-nine-seven-five" (3975). Yet, in the final screenplay, while keeping the reference to Taylor's age, Brent's arrival explicitly but inexplicably changes to 3955, and it remains so in the script for Escape.
Reconciling the two conflicting dates is virtually impossible. For the sake of consistency, The Sacred Scrolls treats the year 3978 as the canonical year of events for both Planet of the Apes and Beneath the Planet of the Apes. All other dates that correspond to either film, including birth-dates, date of death, etc. will be relative to the year 3978.