The expiration time has to do with diaper composition and the warehouse conditions.  A typical disposable diaper will have a shelf life of about 2 years, however diapers made with waist elastic foams will probably last less than a year before there are obvious signs of discoloration on the elastic. This does not mean that the diaper can not be used, it only means it will not look the same as if it was fresh.  After long expositions to heat in a warehouse it is also possible for the hot melt adhesive to fail, this is more sensitive for the elastic adhesives, such as in lateral gathers and leg cuffs, and it is a function of the amount of hot melt used in the manufacture of the product.  Some properties of the standard adhesive tape will also change with time due to material oxidation.