The date of The Tall Men

It sure would be easy to know the date of The Tall Men if, for example, the movie itself says when it happens.

I haven't seen all of the Tall Men, but the IMDB Quote section for The Tall Men(1955) says:

Title card: Montana Territory - 1866. They came from the South, headed for the goldfields... Ben and Clint Allison, lonely and desperate men. Riding away from a heartbreak memory of Gettysburg. Looking for a new life. A story of tall men - and long shadows.

So the movie happens in 1866 right? Not exactly.

It isn't any more necessary for the whole movie to happen in 1866 than it is for the whole movie to happen in Montana Territory. As it happens, the characters head south to San Antonio, Texas to buy cattle and then drive the herd back north to Montana, so various parts of the movie happen in Montana, Texas, and various places in between.

So the opening title says that the first scene in The Tall Men happens in Montana sometime between January 1, 1866, and December 31, 1866. The later scenes happen in the places and times that plot evidence indicates.

In the parts of the movie that I saw the weather was snowy. then the characters reached San Antonio, Texas and warm weather. The weather continued to be warm as they drove the cattle up to Montana.

There are two date possibilities:

1) The snow scenes are sometime in January to March 1866 and the warm weather scenes are sometime in April to October 1866.

2) The snow scenes are sometime in October to December 1866 and on into March 1867 and the warm weather scenes are sometime in April to October 1867.

So did the cattle drive north to Montana happen in 1866 or 1867?