The period-after-opening takes the shape of a number followed by the letter M (for “month”) written in an open jar. It indicates the maximum period-after-opening recommended for which the product "can be used without any harm to the consumer", as specified in the European regulation dedicated.
12 M means the cosmetic product may be used without any problem for 12 months, as from the day its container was opened for the first time. After this period of time, the product may lose part of its properties or microbiological safety, or sometimes only its texture or colour.
This information is important, but consumers still have to play an active role: indeed it is recommended to write down – on the packaging or tube – the date it was first opened, which is the only guarantee for this logo to be useful…
The open jar is mandatory for products which may be used beyond 30 months after manufacturing. As for others, only a date of minimum durability is provided for by regulations.