Water can’t exactly go bad, if what you have is truly just water and it’s in a sealed bottle. There aren’t any nutrients for bacteria to eat, so it’s not like you can get food poisoning. So why does bottled water have a “best by” date on it?
One rumour is that the expiration date is for when the bottle expires. While that’s not technically correct — I couldn’t find any indication that plastic “expires” — it’s closer to the truth. After a few years sitting around, your bottled water may start to taste a bit...plasticky.
The date on bottled water isn’t an expiration date, by the way. Bottles tend to carry a “best by” date instead. Check with the bottled water companies, and they’ll all tell you: that’s for taste, not safety. For example, here’s Poland Spring:
Never fear! You don’t have to worry about our bottled water going bad. (In other words, you don’t have to chug your water as soon as you get it — but don’t judge us if we do.) We do, however, have a “Best By” date to indicate when it may begin to lose its fresh spring taste.
The International Bottled Water Association provides a little more information. While they deny that any significant amount of plastic leaches from the bottle into the water (a claim that seems to be controversial), the bottle is still kind of at fault if your water starts tasting funny after a year or two in storage:
Bottled water and other beverages are packaged in sanitary and highly protective, sealed plastic containers that maintain the quality and freshness of the product. However, plastic containers – whether used for bottled water or other beverages – are slightly permeable, which may allow ambient air gases such as vapors from household solvents, petroleum-based fuels and other chemicals, to affect the taste and odor of your beverage.
To keep water tasting fresh, they recommend storing it in a cool place, out of direct sunlight and away from solvents and other strong-smelling chemicals.