Thankfully the graceful quotes plugin actually had the lines commented out already. Since it was a multi-line comment, there wasn’t a // beforehand. That brought it down to the two plugins. I should have guessed the third one (inlineposts) as the other would have included a second, and redundant, date in the log.
Glad to know this in the future, though, so hopefully I can troubleshoot a bit more before asking for help again.