Quote by Dave__Notts
Someone has already said that use by dates are for perishable foods that need refridgeration, as a rule. Use by dates are a date that the manufacturer uses on food that they have microbiologically tested to prove that the growth of pathogenic bacteria will not reach a level that can harm a person. They usually have a 25% tollerance on that so a day or two out shouldn't be a problem. To sell in a retail shop past this date is an offence but at home most use the tollerance as well as the feel and sniff test. What foods should have use by dates? Usually cooked meats, soft cheeses, ready to eat foods, etc. Most things that contain proteins.
Food items like milk sometimes display use by dates but are not strictly hazardous. If you leave milk......eventually you have yoghurt. Nobody turns their nose up at that.
Dave_Notts
As I understand it, modern milk production kills all the bacteria in the milk - including the good stuff. It's the good stuff that makes yogurt (the stuff in Actimel etc).
If you leave modern milk it just rots. To make yogurt you have to add the bacteria somehow or use unpasteurised.
So, I would definitely recommend NOT eating milk that is obviously past its best.