Use of sour milk for bread making - how long after the expiration date is it safe? My mother uses (and my grandmothers used to use) sour milk in the baking of homemade bread. This was generally  whole raw milk that had soured. 

I'm not so lucky as to be able to get whole raw milk from a source I trust any more but occasionally a carton escapes the thirsty horde at home and goes past its expiration date. If it's only a day or so gone then I'll use it as normal given that I exercise a healthy degree of scepticism anyway about BB dates but if it's any longer then my idea would be that I'd use it in a bread recipe. 

To clarify: the carton is unopened and has been refrigerated since purchase. It is not UHT milk as that's relatively rare here with HTST (72C for 15 seconds) pasteurisation the norm (probably owing to the relatively low ambient temperatures here)

The question is how long after the expiry date is it safe to use milk in this manner?
I'm not so lucky as to be able to get whole raw milk…

Wait, then there's a flaw in your premise. Check the label. Any milk treated using high-heat processes like UHT, pasteurization or ultra-pasteurized milk doesn't sour like it used to in your grandmother's days; it spoils… goes rotten.
Spoiled milk is not the same as soured milk.
The ultra-pasturized milk sold in the supermarket is essentially a dead product, with little to no live bacteria. When it goes bad, it rots and should be thrown out (or you can make plastic with it). Modern industrial pasteurization kills the “good” bacteria that once made sour milk a wonderful thing for cooking and making bread and cheese.
Substituting &quot;Sour Milk&quot;
If you have a recipe calling for sour milk, put a teaspoon of vinegar in a cup of fresh milk and let it sit for a few minutes. That's about the best you can do if you do not have access to raw milk.