Why does bottled water have an expiry date?

Not because the water goes off. Water does not spoil. The date is on the bottle for a mix of regulatory and practical reasons, none of which is about the water becoming unsafe on that day.

The main reason is regulation applied broadly. Some jurisdictions have required a date on all packaged foods and beverages, and water was swept in with everything else rather than exempted. Manufacturers found it simpler to print a date on every bottle than to run different labelling for different markets.

The second reason is genuine, and it concerns the container rather than the contents. Plastic bottles are slightly permeable. Over long periods, and especially if stored somewhere warm or near strong-smelling substances, compounds can migrate both ways — the water can pick up a faintly plastic taste, and odours from paint, cleaning products or petrol stored nearby can pass through the plastic. That is a quality issue, not a safety one, and it is why bottled water should not be kept in a garage next to solvents.

Heat accelerates this, which is another reason not to leave bottles in a hot car for weeks.

Carbonated water has an additional, real limitation: carbonation escapes gradually through the closure, so sparkling water genuinely does go flat.

Still water in a sealed bottle, stored cool and dark, remains safe well beyond its printed date. Official guidance in several countries recommends rotating emergency water supplies every year or two, which is precautionary rather than a claim of spoilage.

Once opened, ordinary hygiene applies — bacteria introduced from the mouth or the air will grow, so an opened bottle left at room temperature for days is a different matter.