I just had one where I didn't use it at all for a few weeks, and it was a few days past the expiration date. This may help, but it wasn't opened yet. My wife was like, "Throw it out!" And I was like no ill take the risk. Decided to have cereal the next morning and was pleasantly surprised it was perfectly fine and was able to use it all within the next 3 days.

Then again, I have had times where i just got it, and 2 days later, it was super gross. Here's looking at you stop and shop store brand milk... got burned twice like that, and I have never purchased it again. I hate that store so much.