There is a purpose for the expiration date, but they won't tell you this. They want the politically correct people to believe that they have to discard the water after the expiration date so that they will buy more, increasing the company's profits. If the water is clean potable water to begin with, there should be no dangerous organisms in it. If it is kept closed, any microbes there will not have any source of fuel, thus no multiplying. Storage in a opaque container will stop the source of energy that is used by algae type life (light). Water can't decompose like organic chemicals such as drugs can, so it doesn't have an expiration date. I filled my water storage in January, when our tap water is odorless. In the summer, our tap water sometimes smells faintly of swamp or chlorine.
The only thing that may happen to water is plasticizers leaching from the plastic container that it is in. I have water that I stored in a 30gal. opaque food grade drum 3 years ago. I checked it recently & it tastes & looks fine. Don't check it too frequently. Each time that you open it is a chance of possibly contaminating it. There are no chemicals in it other than the chlorine used by the municipal water people, so I will have to use clean food safe containers (not the bucket from the garage).