Christian Horn and his family are on a wagon train headed for the New Mexico Territory in 1847. The New Mexico Territory was established in 1850, on land that Mexico ceded to the United States in 1848.
Joe returns to the diner after picking up the rifle (which is now old and dried out). As he opens the door to re-enter the diner, he is carrying the rifle in his left hand but the inside shot of him coming through the door shows him carrying it with his right hand.
When Christian returns to the wagon train at the end, he tells of all the things that will one day be in California, and mentions the word "highways." Being that it's 1847, how would he know what a highway is unless the diner owners explained it to him at some point?