You're not wrong. Distilled fuels like gas or diesel have a fairly short shelf life. You can't just stock it up and hold it for rainy day. Generally speaking, its a good idea to make sure that your fuel is less than 6 months old.
Even untreated diesel is often good for two or three years. I've used it myself. There is an article about a guy who successfully used 15 year old diesel after treating it with a renewer. Microbial growth can be a problem but you can treat that too. Gasoline is much more difficult to store without treatment, especially if any sunlight can reach it.