Thanks.
Just found is on the API website:
"Each bottle of reagent has a Lot # printed on the bottle towards the top. The last four digits of this Lot # are the month and year of manufacture.
Ammonia, nitrate, and high-range pH test reagents will last 3 years from the manufactured date.
Nitrite test reagents will last 4 years from the manufactured date.
Low-range pH test reagents will last 5 years from the manufactured date.
How long the test bottle lasts once it is opened depends on individual circumstances, but as a rule bottles should be discarded within 12 months of opening."
I'm guessing that since I bought mine a few years ago that they're a tad out of date now.....