If one has more than 1 batch of milk stored it is convenient to label breast milk batches with dates. While some may choose to label it with the date it was expressed/pumped. I think it would be easier to put on the "expiry date" instead. But better yet, why not put both dates on the labels?
Note: If it had been thawed from frozen state, it needs to be labeled again before putting it in the refrigerator since it's lifespan is now down to 24 hours and must not be refrozen.
This I think is also a good article with FAQs on breastmilk storage.
What are the best storage containers for breast milk
Standard baby bottles (glass or plastic)
Custom-purpose milk storage bags
How long can I:
Leave my freshly-expressed milk at room temperature?
Up to four hours at 80 degrees F (27 degrees C)
Up to 10 hours at 70 degrees F (21 degrees C)
Up to 24 hours at 60 degrees F (16 degrees C) e.g. in a cooler with ice packs
Store milk in the refrigerator?
Five days at 39 degrees F (4 degrees C)
Store milk in the freezer?
Two weeks in a freezer compartment inside a refrigerator
Three months in a self-defrosting freezer (store away from the door)
Six months in a deep freeze that maintains a constant 0 degrees F (-19 degrees C)
NOTE: You can combine milk from different pump sessions, but cool each batch first and store according to the age of the oldest milk. It is best to freeze your milk in small quantities (2 to 4 ounces depending on the age of your baby) within two days of expressing. Leave room in the container for expansion when freezing, and date each container. As your milk cools, the cream may form a thin layer on the surface. Swirl to redistribute the cream when you warm the milk.
How do I heat breast milk? Microwaving destroys some of the anti-infective properties of the breast milk and may create "hot spots" that could burn your baby. Place the container of milk in a bowl of warm water, or place under the hot water faucet, and heat to room temperature. Your baby may refuse milk straight from the refrigerator, but cold milk is not harmful.
Can I reuse leftover milk? While there is no research on this topic, many mothers refrigerate milk the baby did not finish and use it at the very next feed. If you have any doubt, discard the milk.
How do I thaw frozen milk? Place the container of breast milk in a bowl of warm water. Use the milk immediately or refrigerate for up to 24 hours. Do not refreeze.
Why does my frozen milk smell unpleasant when I thaw it? Lipase, the enzyme that digests fats, stays active when milk is frozen. Some mothers have a high level of this enzyme and their milk smells and tastes soapy when thawed. The milk is not harmful but the baby may refuse it. You can scald your milk before storing it to make the lipase inactive by heating the milk to about 180 degrees F (82 degrees C), or until small bubbles form around the edge of the pan. Chill quickly and store.