We'll give you information taken directly from a few of Japan's government agencies who helped come up with these guidelines, and try to make it as digestible as that slightly-past-its-prime treasure you dug up.
On perishables, like a bento, or sandwich, or milk or any other item with a really short shelf life including food that needs refrigeration, there will be a "Use-By" date and that's the date after which you need to toss that food or risk illness, and this term is all about food safety.
But since we don't include those kind of products in our crates, we'll go on to the info that will actually be useful to you.
The Japanese candies, chips, dried noodles, drinks, and other snacks in our crate will have these 4 kanji on them somewhere, often printed in a white box, with black printed numbers underneath. This word is read as "Shomi kigen"
Those words pretty directly translated mean "Guaranteed Delicious Term", or what we call the "Best Before" or "Best if Eaten By" date here, which is really just the time frame in which the chip / candy / noodle / drink manufacturer, based on internal tests of their product, thinks is the period of peak deliciousness for that wonderful snack.
So this is all about the flavor, baby.
Long story short, as long as the packaging is sealed, and it has been stored somewhere clean and safe and dark, most of these can be eaten for a fairly long time after that date has passed.