Just randomly clicked through a similar posts link and found an article that was (actually) published in 2015 claiming to be published in 2020. Clicking through more random links, it seems that every article that existed when I moved the blog to the new service on May 18th of last year got the timestamps modified to "current time"... so even articles from 2004 (around when I started the blog) are showing up as having been written during an incredibly productive blogging day last year.