I was in the middle of making dinner when this happened. I’m grateful I poured it into a measuring cup first. Thankfully I don’t live too far from another source.
I remember milk staying good almost a week past its expiration date when I was a kid. Boy have the times changed.
That is how expiration dates usually work for anything moist/liquid/perishable. You open it and then you should consume it within a short time. Typically a few days at most, bit more for marmalade.
Reason: food contains microbes/spores. Preservation processes slow down growth, and/or reduce initial amount, but not to zero. Microorganisms in food grow exponentially over time, and the best before date is a statistically determined date by which 99.x% of food samples are still good to eat if unopened. Open it, and you expose the food to the much higher load of microbial life from ambient air and whatever you stick in there (spoon, butter knife, drink from the bottle). Boom, microbial growth explodes and food perishes within a short period.
Same goes for interrupting a cooling chain or exposing e.g. milk to sunlight.