Factual error: In the very beginning of the movie, the narrator says, "Pony Express riders know that one more such attack (referring to the Custer Massacre) would mean a hundred years before another wagon train crosses the prairie." Then they show a Pony Express rider supposedly carrying the news. But the Custer massacre happened in June of 1876, and the Pony Express went out of business in 1861.
Factual error: It opens indicating that Custer is dead. Which would be June 25, 1876. They show a calendar showing a month starting with a Wednesday with 31 days. In 1876 only March started with a Wednesday and had 31 days. When Nathan Brittles is retiring, Olivia Dandridge is attempting to leave the fort before winter sets in, which doesn't fit March.