I fixed it... kinda. Since I was fortunate enough to catch it on the 28th of the same month, I just set the "dayofMonth" variable (near the end of the save file) back to the proper value, along with the "dayofMonthforSaveGame" (it's the only one flagged with this), in both the farm file and the SaveGameInfo files. This won't undo the passage of time of course, so that leaves me with a bit of cleanup:
Minor:
Machines and crops are ready which shouldn't be. That's fine, I know when they're supposed to be done, so I'll just avoid harvesting / turning over until the appropriate date and line that back up.
Animals have been starving and are displeased with me. A minor loss, no biggie, I'll get them fed and over time that'll correct itself.
Several folks I don't have full hearts with haven't been talked to in weeks and are likely displeased. Nothing some gifts can't fix, I'll just get them maxed a week or two later than expected, no problem.
I missed out on some tasks, and new tasks are available instead. Ultimately that's a wash, it is what it is, I'll just roll with the new ones as best as I can.
Major:
Queen of Sauce schedule is goofed up. To be safe I'll avoid watching on Wednesdays, and that should sort itself out soon enough, we'll see if watching on Sundays is safe, if not I'll just catch the reruns next year, it's fine.
Several quests are sitting in the mailbox which shouldn't exist yet. I honestly don't know what the consequences of that one will be. My plan is to not check the mail until the 28th and hope I don't get a bunch of duplicate letters and duplicate quests, if I do I may have to do some more save file surgery.
The Banana tree I planted a week ago is fully grown and bearing fruit. I *could* just wait to harvest it until the time is appropriate. I should, probably. Doing anything else is cheating, right? Man am I ever tempted though to call those Bananas my recompense for the losses and setbacks I have suffered and will suffer elsewhere. I shouldn't, right?
If anything turns sideways, like event date desyncs, or weird forage schedules, or any other artifacts which point to a date counter being used elsewhere in the save file which wasn't updated, I'll post on it as well as what it is if and when I find it. Has anyone done this, and was there anything else they had to watch out for? In general, anything else I should be doing or not doing to smooth things out and ensure that, to the greatest extent possible, I'm picking up where I left off and not just stealing time?
