How do I eliminate the error "Timestamp Mismatch"?
[UPDATE - IS FIXED]  

I fixed this by revisiting WordPress's settings and changing the Timezone setting from "UTC -5" to "Chicago".  Both of these settings reflect the same time zone, so why these two settings are seen as different I can't say, but Civi no longer complains. 

So I guess the lesson is: if you want to get rid of this error and you're showing no actual disagreement on timezone between the server, PHP or WordPress, go to WordPress and pick the timezone that is named for the city you're aiming at.

Hope that helps someone.