There is ALSO an expiration date printed on the gallon bottle. I believe this is what Shiggy is inquiring about, rather than use of the activator.
It's funny this question comes up now, because I have a bottle of Metricide 14 purchased 2.5 years ago, which expired April 2012. And just two days ago I checked it with a Hach GT-1 Glutaraldehyde Test Kit. It's only lost about 15% of its potency. It still grows plants, and kills algae when overdosed, just as expected.
So I'd say expired Metricide is OK.