Ditto. Refrigeration is best.
The number of viable yeast cells starts to decline once a sachet or pack is made. The expiration date is just a best guess for when the number of viable yeast cells will have declined to the point at which a quick starting fermentation can no longer be guaranteed. Storing your yeast under refrigeration will help; ordering yeast during a heat wave may hurt. Starting your yeast in a starter will help get things going and is always advisable, but perhaps more important when dealing with a troublesome must or an old yeast.