I don't think I've been to a Trump county since September 2015. Granted, the majority of my time since then was spent in Australia. Though I flew over a bunch of Trump counties in November when I flew MSP->SJC and back.
EDIT: And actually, even in September 2015, I was just driving through a Trump county, without ever actually getting out of the car. The last time I was actually in a Trump county in which I stepped foot outside was January 2015, when I went to visit friends in Fargo.
Oh wait, never mind, scratch all of the above. I was in Scott County, MN briefly a few months ago, shortly after I moved back to the US. So I guess that was the last time.