Does Milk expire?

The date on fresh milk is a quality estimate that assumes constant refrigeration, and the single largest factor in whether it holds is temperature during the journey home. Milk kept at 4 °C lasts roughly twice as long as milk kept at 7 °C. UHT milk is heated to about 140 °C for a couple of seconds, which sterilises it rather than merely reducing the bacterial load, and that is why it keeps unrefrigerated until opened — after which it behaves exactly like fresh milk.

How to store it

• Back of the middle shelf, never the door.
• Put it in the fridge first when you unpack shopping.

How to tell it has gone

• Sour smell, thickening, or separation that does not resolve when shaken.
• Milk that has soured is unpleasant rather than dangerous; milk contaminated with pathogens gives no warning at all, which is the real argument for pasteurisation.

"Use by" and "best before" are legally different. Use-by is a safety date and should be respected; best-before is about quality, and food is usually fine well past it. Where the two diverge matters, the page above says which applies.