there is a difference between a sell by date and an 'eat by date' if it should not be eaten after a certain date it will say so- the dates you find on cans --many packaged foods are sell by dates- makes the stores rotate their stock--
as for the cake mixes- and foods like that- after some time they will get stale- and not work as well as when fresh---just like lots of stuff- it is better fresh- that doesn't mean it's harmful- it just might not raise as well- or be as flavorful- for (boxed) packaged foods it is best to use it- fairly soon after an experation date---the longer it sits the staler it will become.
and remember --- the dates on foods and the dates on MEDICATIONS are 2 different animals! if you have medication that has expired it may not have the benifits it once had- and you should check with your doctor or pharmacist before taking it.