The expiration date on yogurt makes me laugh the most. I mean... the milk has already been "spoiled" by bacteria, and the pH is pretty acidic; what are we afraid of? I have a former co-worker whose family won't touch anything that's past its expiration date; she kept us pretty well stocked with "old" yogurt most of the time! As PPs have said- Look at it and smell it. If you see anything growing in it, or it smells bad, throw it out. Otherwise, use it up!
There are even exceptions to the "growing on it" thing. I used to work for Hickory Farms decades ago. The first thing we did every morning before opening the store was to check the cheese. We'd look at every piece of hard or semi-soft cheese and if we saw mold on it, we'd use a cheese plane or knife to remove the mold. No problems there- milk is inoculated with either mold or bacteria to MAKE the cheese! I think that most "expiration dates" are on there to protect manufacturers from any liability, just in case someone does eat something that is truly spoiled!