open pot symbol on back of plastic containers

[carlasnook]

Can anyone please tell me what the small symbol on the back of most plastic containers (ie shampoo bottles, cream pots etc...) is, that looks like a 'pot' with an open lid. It usually has a number, followed by a letter, such as 12M. Many thanks.

[NazNomad]

cosmetic products made after 11th March 2005 must have this symbol, and it denotes the shelf life. M= months, although I've never seen a 'D' or a 'Y', though I suspect it would e used in those cases where days or years applied.