Normally, chalk will remain safe and usable even if it is used beyond the expiration date on the label, which is only placed there for the sole purpose of complying with the regulations. Even after the designated expiration date, chalk will not lose its effectiveness, or react differently when exposed to other chemicals.  However, chalk may absorb moisture from its surrounding environment when not stored in a sealed container. This is something you want to avoid. It is best to keep your chalk inside a ziplock bag or in airtight containers before stashing it together with your other lifting equipment. If your chalk gets wet, you can always bake the moisture out to 120 degrees celcius in an oven or simply place it outside and let the sun do its thing.