Time is not what degrades most reagents, so an expiry date is just irrelevant. You suppliers indicate a date because someone (for instance their QA, or Law) instructed them to, so they just put some date and hoped nobody would care about.
If a compound is sensitive to light, CO2, oxygen, ozone... then the storing conditions decide, not the date. And even for some compounds that decompose slowly spontaneously, the storage temperature, the impurities added after opening the container, and so on, matter more than the duration.
So you're asking how to better apply an irrelevant metrics. The ideal answer would be to explain your QA that expiry date is irrelevant, but I understand it can be difficult, especially if your lab must justify itself before judges in the future.