Joe, if it was really square rule joinery you'd see housings at every joint, for example between the tie and post or the post and plate. As your photos don't show any housings I'd have to say it appears to be a scribe rule barn. Unless someone else on this forum can explain it better and show us examples from your photos that prove it is square rule.

So, having said that, it could make the date of the barn earlier than 1820.

One way to investigate age is to look at land tax records with either the town or look at deed records at the county, sometimes the deeds mention structures.

Jim Rogers