[Grammar] Using "must" to speak about the past

[Usernаme]

Hello! I know it's possible to use "must+have been" for the past. When I want to speak about a certain event in the past but I don't quite remember when exactly it happened, will such sentences be correct:"It must have been in 1960s when this supposition was proved by American scientists" or "It must have been in 1880s when the Nine Years War started"?

 

[GoesStation]

Almost. Write "It must have been in the 1880s [STRIKE]when[/STRIKE] that the Nine Years War started".

You could express nearly the same idea this way: "The Nine Years War must have started in the 1880s."

 

[Rover_KE]

Username, there is no need to quote our replies back to us or to make a new post to say 'Thank you'.

Your click on 'Thank' is all that is necessary.

 

[MikeNewYork]

It should also be "the 1960s".