Baking with milk/buttermilk close to expiration date and related shelf life questions
What happens when you bake with milk/buttermilk that is close to it's expiration date? Does the baking process eliminate the need to concern myself with this date? Does the now baked good have a new shelf life of it's own? What is the normal shelf life of a home baked refrigerated cake? Does freezing a cake/cupcake days after being baked extend the life of the baked good upon defrosting it, and if so, for how long?
First, the expiration dates on milk and, especially, buttermilk are waaay conservative. They aren't actually expiration dates but "sell by" dates - which means that if it hasn't been sold, the store should pull it off the shelf. Unless you've left it on the counter at room temperature for half a day or otherwise prematurely aged the product, it can still be good for days, if not weeks, afterward. I've personally used buttermilk that was months past "expiration" date and it was still perfectly good. So learn to use your nose and taste buds to tell if your dairy has gone off, and don't rely on those dates for anything at all. A sniff should be enough to tell you whether it's still ok. Truth is, you can buy a quart of milk that some doofus has left on the cereal shelf in the store overnight, then someone put back into the cooler and it will be spoiled whether it's reached its drop dead date or not.
Second, when you use any dairy in a baked item, the baking process sterilizes the food and so it's pretty guaranteed to be safe to eat. Even if you've used slightly spoiled milk - which I don't recommend, by the way, but it does happen - after baking, the bacteria that caused the spoilage are gone. The "off" taste, however may remain. But you won't die if you eat it.
As for freezing, when you freeze something like a bread, muffin, cookie or whatnot, it's like going into suspended animation. It's exactly as stale when you thaw it out as it was when you put it into the freezer. You can't really reset the clock by freezing. You can, however, extend its useful life so you can keep it almost indefinitely - but it doesn't get fresher. Best plan is to freeze as soon after baking as possible, which will give you a fresh item when you take it out.