I have always wonder ,how do companies know the particular day their product will expire , as in how did they figure it out.

.

..

They use software. When a new product is bought, the quantity and the date it will expire are inputted into the software. It send notification in the form of SMS, email or other means to the company giving some months or weeks notification.
When the expiring date of a new product is closer than the old products they have, they use the new date. If the new product have a farther date, they use the old products until all the old products finished