Unlike with food, there is no expiry date for cosmetics. However, on the back of our skin care products (or on the cardboard packaging), there is sometimes a little logo (an open pot of cream) with a number inside: 6M, 12M, 18M, etc. That is the PAO: Period After Opening. In other words, it is the ideal length of time we can use our skin care product once it has been opened. The PAO was made compulsory by the European Union on 11 March 2015, and follows 2 rules:- It is obligatory on products that can be kept new for longer than 30 months (however, these are extremely rare).- For products that can be used for less than 30 months, the figure on the logo indicates the maximum duration during which the formula will preserve its initial properties.This is defined by a series of quality, safety and microbiological tests.