Hi there. Welcome.
Depends.
If you're script is entirely set in a certain period, World War Two, for example, you could always just open up your first line with the year...
EXT. DUNKIRK - DAY
1940. Troops mass on the beaches...
If you want that information displayed on screen, try...
EXT. DUNKIRK - DAY
Troops mass on the beaches...
Super: Dunkirk, 1940.
Some people say 'super' some people say 'title card' whatever you think works best.
If you're cutting back and forth between times, like the opening of Saving Private Ryan for example, put it in your general description like in the first example to keep things clear. That's the most important thing - clarity. For that reason, I wouldn't put it in your scene headings because it'll just get cluttered.
Hope that makes sense - it really depends on how you're planning to use it.
