Different manufacturers deal with expiration differently. For example, API doesn't print expiration dates on their bottles, but the date of manufacture. Here's a recent post regarding the coding on API bottles...
API Test Bottle Expiration Date Question
On the other hand, Salifert test kits give an actual expiration date on the box. But on the bottles inside the box, lot numbers are given. The lot numbers *appear* to be a date of manufacture.
Would I use the kit after the expiration date? Depends on what test we're talking about and how sure I am of the value it's reporting. If your test kit is old and you're getting numbers that are showing a major shift in parameters - for no apparent reason - then it's probably time to get a new test kit before making any changes. My magnesium kit is nearly a couple years past it's expiration date - but the numbers it gives me make sense and I have no reason to doubt them... so I just keep using it!
Regarding your original question about parameters, here's a little chart:
These are good levels to shoot for AFTER you do what cmor mentioned in his post!