The scenes of cities being attacked from the air with conventional and nuclear weapons are lifted from Invasion, U.S.A. (1952).
The scene on the screen of the spaceship on the launch pad was lifted from the 1964 movie The Time Travelers
When the time-portal starts misbehaving, the rat-spider-crab from 'Angry Red Planet' (1959) is briefly glimpsed on the screen.
The film takes place in 1968 and 6968.
It was very economical to have the scientists only visit the primordial age of Earth themselves, which would just require the building of some jungle and cave sets, instead of massive pirate fleets, World War II armies, and ancient kingdoms.