"The Unvanquished" 12 March 1963.

I suspect that the episodes of many televisions shows are not intended to happen fictionally happen in the order that they are filmed or broadcast.

Thus the creators of those shows might not worry whenever a show from a later season has an earlier fictional date than a show from an earlier season. Those creators might not intend to make episodes in any sort of fictional chronological order and thus might not mind that the fictional dates of episode sort of zigzag around instead of steadily getting later and later.

In "The Unvanquished" 12 March 1963, there are some problems with Arapahoe Indians vs bigoted locals. Rafe tries to stir up a lynch mob, saying that they showed the Arapahos who's boss 5 years ago at Sand Creek. Diamo refers to white treachery at Sand Creek.

There are probably many "Sand Creeks" in the west, and there were a few thousand battles, fights, & skirmishes with Indians in the west. But a fight that was big enough that Rafe claimed it taught a lesson, and a fight that Diamo said was an example of white treachery, and a fight that involved Arapahos, and a fight at a place called Sand Creek, narrows it down a lot.

So it seems probable that Rafe & Diamo talked about the notorious Sand Creek Massacre on December 29, 1864. The victims were mostly Cheyenne but included some of their Arapaho friends and allies, so Tah-Za's wife and Diamo's mother could have been killed there.

If November 29 is approximately eleven twelfths of the way through the year, the Sand Creek Massacre happened about 1864.917. If "five years" were 5.00 to 5.99 years, "The Unvanquished" would happen about 1869.917 to 1870.9167. So the fictional date of "The Unvanquished" is probably intended to be sometime in 1869 or 1870.

In "The Unvanquished" the Arapaho are moving north to a new reservation in the Powder River country. The Southern Arapaho eventually united with the Southern Cheyenne to form the Cheyenne and Arapaho Tribal Nations, a tribe with a reservation in what is now Oklahoma. In 1878 the Northern Arapaho were "temporarily" given space in the Wind River Indian Reservation (west of the Powder River Country) in Wyoming owned by their old enemies the Eastern Shoshone and officially became half owners of the reservation in 1936.

So 1869 to 1870 on, or 1878, seem to be the possible dates for "The Unvanquished" if it happens in the historical instead of fictional west.

See discussion of the fictional dates of "Circle of Fire" and "The Pass", broadcast years earlier, for comparison.