Have you noticed this on the back or your skincare and cosmetics? Basically, it's the 'Good For __ Months' after you have opened a product. 6M = 6 Months, 12M = 12 Months etc.
When was the last time you cleaned out and purged old makeup and lingering skincare? It might seem like a daunting task if you're just discovered that cosmetics have an expiration date but I have easily broken down what and when it needs to go.
No matter how much product is remaining in the vessel.