The fictional date.

Melinda tells Paul Regret her husband was killed 4 years earlier at the Battle of San Jacinto which happened on April 21, 1836 in real life.

If there are any movies, such as Man of Conquest (1939), a biography of Sam Houston, that give the date of the Battle of San Jacinto, and if The Comancheros is considered to happen in the same fictional universe as such a movie, then The Comancheros would happen in or about 1840.

Naturally there are many anachronisms in the movie which make it look like it could have happened 30 or 40 years later.

Ed McBain spent 5 years in Yuma Territorial Prison, for example, which was hard to have done in 1840 since the USA didn't acquire the land around Yuma until the Gadsden Purchase in 1854 and the Yuma Territorial Prison opened July 1, 1876.

Added June 19 '19. The Opening title card says: "1843 Louisiana". So Melinda must be very bad at math or else The Comancheros happens in an alternate universe where the Texas War of Independence happened in 1839, not 1836.