I found the pref file (did a find for "clock") and dragged it to the trash, then rebooted. The time was still wrong, of course, on the "new" time menu bar, but once I changed it and rebooted, it has held ever since. Just got up and booted and it still holds. For those who trashed the file and rebooted and still have the "old" time and date, try correcting it one last time.
Thanks to the moderator or whomever suggested trashing that file.
I sent a big missive to Apple about this. Hopefully they'll start seeing this as an issue to deal with.