asking the Col. about a timeline, Col. says Twenty Two Hundred Hours... Columbo asks What time is that... Earlier when he was settling into his room in the Barracks, he told the Cadet "when I was in the Army..." so he would have known what 2200 hours was, why did the writers do that?
As we learn in Swan Song, Columbo served in Korea, not in the Military Police, but, as he says, "KP (kitchen patrol) mostly." Since he did a lot of time doing KP, we can assume that Columbo wasn't very good in the military and never bothered to learn military time.
Yes, it could be the fact he was mostly in KP or it could be another example of the shrewd lieutenant deliberately playing dumb which is one of his most notable character traits.