Most cosmetics carry a small icon of an open jar with a number followed by the letter M. It answers a narrower question than most shoppers assume.
What the number counts from
The figure is the period after opening. It starts the moment the seal is broken and the formula first meets air, skin and whatever is on a brush.
It says nothing about how long an unopened product lasts in a warehouse or a drawer. A sealed package is a different situation with different risks.
This is why two identical products bought on the same day can be in different condition a year later depending on when each was opened.
Why preservation is the limiting factor
Any formula containing water can support microbial growth. Preservative systems are designed to control that growth for a defined period under expected use.
Stability testing exposes the formula to repeated contamination and temperature cycling, and the period after opening reflects how long it held up under those conditions.
Anhydrous products such as pressed powders and oil-based balms have far less water available, which is why their stated periods are typically much longer.
How United States labeling differs
American cosmetic labeling rules do not require an expiration date on most products. Manufacturers may include the open-jar symbol voluntarily, and many do because they also sell internationally.
Sunscreens are the notable exception. They are regulated as over-the-counter drugs in the United States and carry an expiration date unless stability data supports omitting it.
So the presence or absence of a date on an American shelf reflects the product's regulatory category more than the manufacturer's confidence in it.
Why real conditions shorten the period
The stated figure assumes normal storage. A bathroom counter beside a shower is warm and humid, which accelerates both chemical breakdown and microbial growth.
Fingers introduce far more contamination than a pump or a spatula. A jar dipped into daily is under more load than a tube of the same formula.
A car in summer is the harshest common storage location, and heat damage to emulsions is often irreversible even after the product cools.
What separation and smell actually indicate
An emulsion that has split into layers has lost its structure, which usually means the balance of oil, water and emulsifier has broken down.
A change in odor, color or thickness signals chemical change. It is not a reliable test for contamination, since microbial growth can occur without any visible sign.
Eye products deserve the strictest handling, and any product that causes stinging, redness or swelling should go to a physician's judgment rather than back in the drawer.