How long milk will last after the expiration date depends. I have had milk last for almost a week past the expiration date, and at other times I have had milk go bad before the expiration date. That may have a lot to do with the conditions the milk encountered before it was even brought home from the grocery store.
Milk keeps longer if it is kept at a temperature of 45 ºF or below and not left out at room temperature for long periods of time. When you buy milk in the summertime, for instance, there is a chance that the milk may have been allowed to sit out in the heat for a time before it was put into your grocer's refrigerated section. At home, use just what you need and immediately return the rest to the refrigerator. Do not let milk sit out at room temperature for long periods of time.
Harmful bacteria can grow rapidly in milk above 45 ºF. However, temperatures well below 40 ºF are necessary to protect the milk's quality. It is critical that these temperatures be maintained through warehousing, distribution, delivery and storage.
You can extend the life of milk by freezing, but it may separate or be slightly grainy when thawed. Frozen milk works best for cooking or putting on cereal or in coffee, but some people find it's still okay for drinking.
You should only freeze milk before the 'Best before' date. It should be used within 1 month of freezing and should be defrosted slowly in the refrigerator, this will take about 24 hours. The defrosted milk should be used within 2-3 days. It is advisable to give the milk a good shake or stir before use. Skimmed or semi-skimmed milk freezes slightly more successfully than whole milk.
Freeze milk in plastic freezer containers or special freezer-proof glass jars. Leave some extra space at the top since milk expands during freezing. If packaged in a wide-mouth container, leave 1/2-inch head space for pints and 1-inch for quarts. If packaged in a narrow-mouth container (such as jars), leave 1 1/2-inch head space for either pints or quarts.