In what year does Once Upon a Time in the West take place?

A lot of western movies happen in very vaguely defined times and places.

Because guns, costumes, and props are generally selected by the costume and prop persons out of items already used in other westerns and stored at the studio or rented from prop and costume suppliers, there are many examples where a movie has a directly stated date which is different from the time period when various props and costumes were in use. I consider scripted statements or writen dates to be far more important clues to the intended date than props selected by the prop people.

In many cases the props and costumes used in a western movie come from decades after its stated date.

In the case of The Comancheros (1961) a title card at the beginning give the date as 1843, and a woman says her husband was killed at the Battle of San Jacinto 4 years earlier (which woud make the date 1840 - I guess she wasn't good at simple arithmatic), but the costumes and weapons are more consistent with a period 4 decades later.

https://moviechat.org/tt0054757/The-Comancheros/5ca18b1f51cfeb1290833d8c/The-fictional-date[1]

One famous western is Duel in the Sun (1946). At the begining a narrator says it happens in Texas in the 1880s. The date can be narrowed down by the fact that a railroad is built through West Texas during the film. Except that the railroad cars are labeled with the name of a fictional railroad which might have laid tracks at a fictional time. I also note that the style of the cavalry standards and guidons indicates a date years after the railroads reached west Texas - but movies almost never use the correct cavalry flags for their fictional dates.

https://moviechat.org/tt0038499/Duel-in-the-Sun/5a11d43839d0810012ca0f0e/What-is-The-Fictional-Date-of-Duel-in-the-Sun[2]

The TALL Men (1955) starts in 1866 and has a cattle drive in 1866 or 1867. But the weapons are 1870s types.

https://moviechat.org/tt0048691/The-Tall-Men/58c7221e5ec57f0478eb91b2/Date-not-even-close[3]

https://moviechat.org/tt0048691/The-Tall-Men/59e11aed4b0d030012845d76/The-date-of-The-Tall-Men[4]

I know a bit about the hstory of the Indian Wars in the west, both the real west and the reel west.

And I have often commented on how various westerns depict war with a specific tribe when there was actually peace, or peace when there was actually war, or put cavalry regiments hundreds of miles from where the were really stationed at the times of the movies.

In The Savage (1952), for example, a soldier says he has been in the army for 20 years since the Civil War, and thus the date should be about 1881-1885. But the title character, a white man raised by Sioux, should have been about 18 to 20 at the time of the Great Sioux War of 1876-77, and thus he should have been old enough to fight then. But the sioux wonder whether he will fight for the Sioux or for the white men, as if the Great Sioux War never happened in The Savage and he never had a chance to show his loyalties.

https://moviechat.org/tt0045123/The-Savage/5d0d59193e04835683e40cc0/The-Date[5]

So if you want to date Once Upon a Time in the West (1968), you should look for evidence which would probably have been written in the script instead of left up to the prop person to find or make. If a character is a veteran of the US Civil War, for example, does he look like a young man or an old man? Does someone read from a newspaper and state the date? Is someone acused of committing a crime on a specific date?