Shelf Life of Diesel Fuel

I recently found (in my garage) a half gallon of diesel fuel that has been stored in one of those red plastic gasoline containers, (it's only been used for diesel fuel though). The cap has been tightly closed on it too. I figure this fuel is about a year old. #1-is it still OK to use? #2 What is the shelf-life of diesel fuel in a sealed container? #3 Does it ever turn into something not useable-like gasoline does into varnish?