The use before or sell by dates don't mean much as I found out. The first part of Dec, I bought an 18 pack of Yoplait yogurt at Sam's with a sell by date of Dec 25. It had 6 cups each of blueberry, strawberry, and peach flavors. A week after I bought it I had one of the blueberry, and it had a little bit of a zip to it, but I thought it must be my tastebuds, since I had eaten some of the other 2 flavors and they were fine. Then several days later I went to get another blueberry, and I noticed the cup was bulging. I opened it and I could hear the gas escape when I did - it was fermented. All the rest of the blueberry cups were the same way and it was still 2 weeks to the sell by date stamped on the top. The strawberry and peach were fine - no problem with them.
I called the customer service number listed on the cup and reported it, and the girl took all kinds of info - wanted to know if I got sick from it or anything like that. Then in the mail a few days ago, I got a letter of apology, a check for $10, and $10 in coupons toward the purchase of more yogurt.
It's a good thing that I didn't let Sparky lick the cup of the one that didn't taste quite right - it might have made him sick. He loves yogurt and always has to have a bit of mine.
So anyway, even tho the date says that it's good, obviously you still need to be quite cautious.