If an on-line retailer has product on the shelf for six months how does that affect the manufacturers twelve month guarantee?

If an on-line retailer has product on the shelf for six months how does that affect the manufacturers twelve month guarantee?
The warranty is valid from the date you purchase the item PROVIDED you can prove when you bought it. If you have a dated receipt, then the warranty is valid from that date because the purchaser has no way of knowing how long the item has been sitting on the shelf.However, if you cannot prove when you bought the item, then many times they will revert back to the manufacture date for the warranty because they have no other date to go off of.This comes up very often when referring to Hard Drives and was talked about in a Maximum PC magazine article a few months back. It all hinges on you keeping your receipt and being able to prove when the item was purchased. Some companies' customer service representatives do not understand how this works and many times you have ot educate them on the facts or talk to a supervisor.