I dont know for sure as it relates to what we use it for, but that date is probably based on its intended use as a sterilizing agent and also takes into account the life of the activator that comes with it (unless they have separate dates, obviously)
For use as a sterilization agent, once the activator is introduced, it has 14 day shelf life if you have Metricide 14 or 28 day if you have Metricde 28.
You are not supposed to use the activator for aquarium use, so the 14 or 28 day shelf life doesn't apply. In tank water, Glut, has 10 hour half life.
In an unopened bottle, Glut, has a few years shelf life as marked by the expiration date in the bottle. I think you are still ok to use it even though it has expired by a few months. Once opened, however, its shelf life is shortened due to exposure to atmospheric contamination. Does anyone has an Excel bottle that says how long it ought to be used up once opened?