Distilled water, unlike most other types of water, does indeed have an expiration date. This is mainly due to a process called oxidation. As distilled water is essentially free from dissolved minerals and other particles, it can more easily come into contact with oxygen molecules from the air which leads to oxidation. Since this process can affect both taste and odor (as well as corrosion in metal containers) it's recommended that this type of water be used within 6 months for best results. It's also worth noting that exposed distilled water has an increased chance of microbial contamination due to the lack of dissolved minerals which act as a natural preservative (which is why bottled/unopened distilled should never expire).