I don't know where I first read this but it has always stuck with me and I am surprised how few people are aware of it. I have even had some cosmetic sales people not know this little golden nugget of information so I am going to pass it along to you.
If you turn over almost any cosmetic product you will see a symbol that looks similar to this:
The number before the M will change and this is what you need to pay attention to. It is called different things by different people, but regardless it means the same thing. The most common term is Period After Opening (PAO).
It indicates how long the product is safe for use after opening. “12M” in this example means that the product will be good for 12 months after opening. The quantity is typically in “M” for months, and the number of months can vary. If you know you’ve bought it before that time frame or you can’t honestly remember when the last time you used the product was, odds are you should toss it! In the future, you should write the date it was opened on the bottom of the jar using a permanent marker, I keep a silver sharpie in my bathroom because it works on both white & black packaging and most other colors!
Here is a great articles if you would like to read more about this topic: