expiration date on butter
From a food safety standpoint, it should be fine, but unless it's been _very_ tightly wrapped, it's probably picked up odors sitting around for so long. Butter is labelled with a very long shelf life (usually around 3 months), so yours has been sitting around somewhere - on a shelf or in your fridge - for a long, long time.
Personally, I'd probably toss it - chances are slim that tastes anywhere near as good as fresh butter. If you bought it recently, remember to check dates in the future; if you've had it that long, freeze most of your next package immediately. As long as it's very (very) tightly wrapped, it will keep well in the freezer for a very long time. I wrap very well in plastic wrap, then put that into a freezer baggy and that keeps it tasting and smelling pretty fresh for several months at least.
Long frozen butter might not be your first choice for a dish where the butter flavor will take center stage, but it'll be just fine for everything else. It doesn't matter how you defrost - in or out of the fridge - obviously you don't want to nuke it or soak it in warm/hot water if you're going to use solid butter....
To clarify, there are two ways something can be "not good" -- one is simply whether it's retained its quality, which you can tell by smelling or tasting. If it doesn't smell or taste good, it isn't good.
The other way something can be "not good" is if has spoiled, that is, it's acquired bacterial contamination that will make you ill if you ingest it. Unfortunately, you can't tell this by look or smell, so with foods susceptible to spoilage, it's better to discard them if you have any questions.
Butter doesn't spoil in that sense, so whether it's good or not depends entirely on whether it still smells and tastes good to you.
Finally, "expiration dates" are completely meaningless when it comes to whether something is "good" or not. There's no standard for what those dates mean; they are designed primarily for letting the distributor and retailer know when stock is getting old and should be replaced. And of course, to get consumers to throw out perfectly good food and buy more.
I have used butter past the expiration date, but I keep my butter in both the freezer and fridge.
I buy 1 lb of butter, I put 1 cube in the fridge, the rest in the freezer inside a freezer ziplock. When I am down to half a cube in the fridge, I put a frozen cube in the fridge. By the time I need it, it has defrosted.
Regarding your "expired butter", I believe it depends on how you have stored it. If you keep butter at room temperature, the butter may develop an off taste. If the butter has been in your fridge, wrap it to protect from freezer burn/smells and freeze it.
Expiration dates for butter are estimates unlike dates on milk which are required by law in some states and are fairly accurate in predicting when the product will go off. If the butter looks good, smells good and tastes good it probably won't hurt you. If otherwise, toss it and make sure your next package of butter has the furthest expiration date possible. These are usually placed in the rear of the butter section 'cause the stores want to get rid of the older packages. Good luck!Jon - who is an expiration date junkie and will often pass up some goodies from Europe because I can't understand the codes they use! :)