I went to the "Potable Aqua" website.
Hope this helps!

WHAT IS THE SHELF LIFE OF POTABLE AQUA TABLETS?

Unopened bottles of Potable Aqua tablets, when maintained under controlled temperatures between 60 and 86 degrees F (15 to 30 degrees C), should remain effective up to four years. Exposure to heat, humidity, moisture, and air will reduce the effectiveness of the tablets. We recommend that an opened bottle not be kept for more than one year. Keep the cap tightly sealed.

Although this product is not required to have expiration dating, an explanation of the lot numbering system is provided for your convenience. This lot number is imprinted on the label of the bottle. Example:
403127

This means the product was manufactured the 4th month in the 2003 and was the 127th batch of product made. Under normal storage conditions, this product would be effective for at least four years for an unopened bottle and one year for an opened bottle.

If in the field, check the tablets appearance to determine if they are still effective. If the tablets are gray or dark brown in color, they are still likely to be effective. If they are light green or yellow, it means they are probably no longer effective. The tablets are very sensitive to moisture. Take care that moisture does not get inside the Potable Aqua bottle.