Timeline

I just finished watching it and it and my copy (dubbed VHS) made no reference to the 1980's. Judging by the human astronauts' reactions (and name recognition) when seeing Monster Zero (Ghidra) appear on Planet X, they instantly knew which monster it was. If the film had taken place in the 1980's, then they would have to remember a dozen or so ridiculous giant monsters that took turns attacking Japan in the past thirty years. They point out Ghidra right off the bat. Also considering the times represented in the subsequent sequels, it appears that Godzilla and Rodan took on King Ghidra (sans Mothra) for the 2nd time in 1965.

[deleted]

Many of the Showa films supposedly take place sometime in the future, yet the chronology only fits in the order the films were made. So if you try to place them in "198X" or whatever it says in the movie, you get a big, jumbled mess. If you watch them in the order they were made, they fit together nicely.

So, ignore whatever year the movie supposedly takes place in. It takes place when it was made, 1965.

All of the Showa movies (save for Destroy All Monsters) can be assumed to take place in the year they were released. This movie is said to be set in "196X" in the beginning of the film, but we can assume it's specifically 1965 given the fact that its events are clearly set after Ghidorah, The Three-Headed Monster and probably before Godzilla vs. The Sea Monster.

I don't see how this movie could take place in the 60s when it was made or in the 80s, because today in the year 2023 we don't have the technology to send a space ship with astronauts in it to a planet past Saturn. This movie has to take place way in the future