Here's my rule, and it applies to gear, as well as food, Rx pills, and even the amount of time between oil changes in the vehicles....
In the USA, food and drug mfrs are subject to lawsuits for ANYTHING. So they make their expiry dates early, and overstate all warnings and dangers. Use common sense: Rule #1 is store whatever it is properly! Don't keep it where there are radical temperature changes, or where it might freeze (unless it's supposed to be frozen of course). I keep mine near the floor, in a safe, where there isn't any light and there's the least change of temps in the building. If a vial has a mfg date of Jan 2010, and it expires Jan 2013, I'd use it in August of 2013, and maybe still in Jan of 2014. If it's mfd Jan of 2010 and expires June 2010, I'd probably not use it much beyond September 2010. And not if there's something weird like a color change, or pressure inside, or particles, etc etc. So maybe I'll happily use something that's 25-30% beyond the expiry date, as long as I have owned it since new and know how it was treated. On the other hand, it also depends on what happens to the substance when it DOES actually break down or fail; if it just peters out, and becomes inert or useless, no problem. If it turns poisonous, then obviously I take that into consideration when fudging the expiry date. A quick Google search should reveal what happens to any particular substance when it breaks down. Same logic with car oil changes, if the mfr says every 5k miles, I'll go 8K miles if the car is garaged, and driven at a constant highway speed most of the time, on long highway commutes. If it's a short-trip farm-to-town runner stored outside where it goes from 30 at night to 80 in the daytime, I'll probably change oil at 2K miles.
Common sense will usually keep you in good shape here.
Charles