It's not completely unimportant, but it's not very important (within reason). Manufacturers guarantee 100% potency until the expiration date (usually two years after manufacture). So these Vit D tablets are guaranteed 100% potent until May, 2020. If they've been 100% potent for TWO years they are not suddenly going to drop to 0% the day after they expire. They will very slowly begin to lose potency. Just keep them in a cool, dry, dark place in their sealed containers. I've spoken to some pharmacists about this and they say that if you keep them in the right conditions, they will likely only lose a few % over the two years after the expiration date. So, instead of being 50,000 they might be 48,500. That's fine for my purposes. And there is no danger, either. Only a few drugs (certain antibiotics become dangerous after the expiration date, not vitamins. Anecdotally, I took a 12 year old Vicodin last week and it worked just fine!