The period-after-opening symbol or PAO symbol is a graphic symbol that identifies the useful lifetime of a cosmetic product after its package has been opened for the first time. It depicts an open cosmetics pot and is used together with a written number of months or years.
The time period is most often represented compactly as a number of months, followed by the letter "M", as in "36M" for a period of thirty-six months, written either onto the front side of the depicted pot, or to the right of it. The letter "M" is the initial for the word month not only in English, but also in many other European languages. It is also used in the ISO 8601 duration notation.