Scientists enzyme degradation comment might have meant that it certainly isn't just a simple % error per month thing. This stuff behaves a bit strange when compared to most school chemistry reaction speeds.
First off, those test strips have really wide error margin allowed, iirc somewhere near 20% +/- of reading at the "healthy" level that is to be expected and even more error in the very low and very high concentrations (reading the instructions will tell you), where you only need to be alarmed and it doesn't really matter exactly how bad it is, maybe ?
No one prevents you from mixing up some calibration fluid with a known glucose content and compare the measurement results of fresh and expired strips.
You might assess the risk to for yourself and decide that the error is not a threat as your blood sugar regulation system works well, then go on and use the strips with the known hack whose existence I consider due to the link posted above.
Hint: Your (healthy) blood sugar level reading is the output of a already regulated and compensated system, so it doesnt mean a lot to begin with... I would like to add something to the large capacitor of scientists model: A voltage regulator with source and sink capabilities May I point you to lactate measurement? This stuff is way more interesting, a rise when in rest is always a bad sign...