Does near expiration date milk continue to "age" in a cooked dish?

You have either a "Best before" or a "Use by" date.

• If it's an unopened pack with a "Best before" date, the milk should still be perfectly safe even quite some time after that date (it's sterilised or UHT treated) in the unopened package.

• If it's an unopened pack with a "Use by" date, it depends on how the pack was stored: a "Use by" date is for raw or pasteurised milk, which should always be kept refrigerated. And normally, the milk should be used on or before that date.

If the pack has been opened, you should be able to keep it for a few days at 4°C (refrigerator), but not beyond a "Use by" date.

I use as a rule of thumb "don't use it if it smells off". If the milk still smells good, when using it in a casserole that is baked, the leftovers should keep normally. In a cheesecake mix, if the milk isn't reheated, I'd be more careful, and not go beyond a "Use by date", or 3-4 days after opening the package.

And the "going off" isn't a matter of aging, but of microbial growth. If it's lactic bacteria, you'll finish with yoghurt; if it's something else, all bets are off...

In the US, most refrigerated, pasteurized or ultra-pasteurized milk products have a 'sell by' date rather than a 'best by' or 'use by' date. (I can't say for sure that this is the case everywhere in the US, but it seems to be predominant in most areas.)

With milk and other dairy products, the date (of any type) is just a guideline. You can't guarantee that because the date is good, the milk will be. Or because the date has passed, the milk will be bad. There's no magic clock that makes it go bad at a given time. The most important thing is how the product has been handled, all the way from production to consumption.

From NOVANEXT article about milk dating:

Potentially harmful bacteria are destroyed in the pasteurization process. They can’t spontaneously regenerate, so old milk stored in the fridge doesn’t present an increased risk of foodborne illness.

So what is it? Often, the date is set for around three weeks after pasteurization, which is when the milk will begin to taste off. After reacting with oxygen and light, the fat in milk becomes rancid or stale. The distinct sour taste of aged milk is a byproduct of harmless lactic acid bacteria that feed on the sugar in milk. “It’s more of a sensory defect that we don’t like, but you’re not going to get sick on it,” says Randy Worobo, professor of food science at Cornell University.

Essentially, the issue here is about quality, not safety. The best guides you have are your senses. If the milk looks okay, smells okay, and tastes okay, it will be okay to use. With milk, if I am unsure, I pour a little in a clean cup, eye it, smell it, and taste it. (Remember that the top of a milk carton or bottle can smell bad even if the milk is okay.)

So, to answer your questions, if the milk is okay when you prepare your dishes, it really doesn't matter what the date is on the carton. It won't change how long your food will keep after preparation.