The fictional date

Colonel Marsden says that the Civil War has been going on for four years.

Marsden would have said "four years" if:

1) He meant the war had been going on for 4.0 to 5.0 years, thus making the date between April, 1865 and April, 1866.

2) He meant the war had been going on for 3.5 to 4.5 years, thus making the date between October, 1864 and October, 1865.

3) He meant the war had been going on during all or part of four different calendar years - 1) 1861, 2) 1862, 3) 1863), and 4), 1864 - thus making the date between January 1, 1864 and December 31, 1864.

The captain in command at Fort Shallon says that if they hold out until the snows come they will be safe from Red Cloud's warriors until the spring when the war would be over and the army could send reinforcements.

The transcontinental telegraph would quickly send news of the Rebel surrenders in April and May, 1865 to every telegraph station in the west, and then it would take just days or weeks for mounted messengers to bring the news to even the most isolated frontier forts. Every soldier in the west would know the war was over long before the fall of 1865.

The captain's statement is consistent with interpretations 2 and 3 of Colonel Marsden's statement, therefore the fictional date of The Last Frontier is fall, 1864.