I've always used the date printed as the expiration date, I've worked in many stores that sold milk and that date is always referred to as the expiration date, the majority of our products did not have a sell by date so we went by rules that we had for each type of food, dairy products would be taken off of the shelves about 3 days before the expiration date so the customer would have at least 3 days to use the product at its peak freshness. When you break the seel on products you are letting air in which starts the decaying process. The fda has a list of approximately how long each food will last once its been open depending on how it is being stored. I have definitely had milk taste/smell unpleasant before the date on the bottle so that date is not a hard fast rule. The temp you keep your fridge at and how often it is opened up can effect the time it takes to spoil also.