What about the sell by date

I have been selling Himalayan rock salt now for quite some time. Today on the company site i order from i read some information about the salt i was about to order. It seems that the various salts from the company i use have a use by date.Why i ask when they have been taken from the grounds where they have been for Millions of years.

 If the salts have been in the ground for Millions of years Why do they need a sell by date. If they have lasted for Millions of years why after the sell by date should they be thrown away???