Although bottled water has an expiration date, it doesn't actually go bad. Why is there an expiration date on a product that doesn't go bad? This is because New Jersey requires all food and beverages, including water, to carry an expiration date on its packaging. It doesn't matter if you don't live in New Jersey... your water may carry an expiration date anyway to make it easier to standardize packaging. Some bottled water only carries its bottling date or a 'best by' date. These dates are helpful because the flavor of the water will change over time as it absorbs chemicals from its packaging. The flavor will not necessarily be bad, but it may be noticeable.
Leaching of chemicals from packaging is a health concern, but as far as toxic chemicals go, you can get exposure to most of those chemicals from freshly bottled water as well as bottled water that has been on the shelf a while. A 'plastic' taste is not necessarily an indicator that the water is bad; absence of an unpleasant flavor does not mean the water is free from contaminants.
While algae and bacteria will not grow in sealed bottled water, the situation changes once the seal has been broken. You should consume or discard water within 2 weeks after opening it.
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