[quote=@Kelewen]We can say 75 if you want to give it an exact year, but I'm going for a more cinematic bent than pure historical.  ;)[/quote]
I like to know the year too. In one of my CSes, I gave the birth year of a child as 1861. Now I know the child is 14, rather than 12 or 13.
Also, the 10th Cavalry was created in the late 1860s. The 10th & 9th Cavalry and the 24th Infantry were all black regiments made up of freed men and former slaves. This means, Billy could have served for four or six years before heading out on his sojourn.