If I promise to pay you for a product by 31 December and I haven't paid you by 10 January, my payment is 10 days "overdue". If I promised to phone you tomorrow and I don't phone you until 10 days from now, my promised phone call is "10 days overdue".
In Canada, if a container of milk is purchased at the store, it will have an expiration date so "days before expiration" is correct if the milk is still "good". If the expiration date has been passed, then it would be referred to as "past due", not "overdue". The phrase "past due" simply means that the date has been "passed". So, if I buy the milk on September 10 and it has an "expiration date" of September 20 and I don't drink it until September 21st, it is "past due" by one day.