All kits expire. I use salifert for my tanks for the most part. I also use a very expensive photometer 25 by Palintest.
I run a waterpark and use the pooltester for checking chemicals in the pools. It also has tests for most, but not all of the tests that I run on the tanks.
So far the Salifert tests have been the most accurate, when compared to the Photometer. Expiration dates are printed on the box. Sometimes it is a sticker applied at manufacture.
Most all these tests are DPD based, color strips, or dip strips are not to be trusted. Everyone see's color slightly different.
When the county comes out to inspect my pools in the summer, they use my tester. Far better than what they have...