I think I checked the FDA site, but it was only partially helpful. IIRC, it did distinguish "use by" date from "sell by" date, but it didn't clarify much about what "use by" really means. Does it mean the product is unsafe to use the day after that date or just that its nutritional value and maybe safety decline after that date.   I think they took what I consider an unnecessarily 'conservative' stance, such as insinuating that maybe you should throw out a product on Sept. 15 if the "use by" date was Sept. 14 -- no matter what the product was or how it was stored.
In other words, it wasn't very detailed.

As for the caregivers and the women who care for her, she is 90 and I think at least a couple of the caregivers (especially the ones who cannot converse with her in any detail in English) think they are doing "what's best for her" by disregarding her instructions about throwing things out.   She has told me and a couple others that she wants people to check with her before throwing things out, but at least one caregiver had disregarded that instruction. I think as long as that caregiver is thoroughly convinced that food "has expired" a month or two after it's put in the freezer, 3 days after a package has been opened and put in the fridge, or a few days after the "sell by" or "best by" date on a package, she will probably continue to act on what she thinks is best for the client. Same goes for one other caregiver whose English is marginal.   I think that educating them on what these various dates really mean would go a long way -- provided the information came from a source they trusted, such as a scientific publication.