Most bathroom cabinets contain at least one bottle whose age nobody can account for. It looks fine, it smells approximately right, and the reasoning is that skincare is not milk. That reasoning is half correct, and the half that is wrong is worth knowing, because a product past its usable life is not usually dangerous so much as pointless: the active you paid for has degraded and the preservative holding everything together is running out of authority.
Two dates, two questions
The date printed on the carton is the shelf life of an unopened product stored sensibly. It answers: how long will this remain what it was while sealed.
The small symbol of an open jar with a number and the letter M, so 6M or 12M, is the period after opening. It answers a different question: once air, fingers and bathroom humidity have been introduced, how long is this reliable. The clock starts the day you break the seal, and it overrides the carton date. A product with two years left on the box and a 6M symbol is a six-month product from the moment you open it.
Neither symbol is a hard cutoff. Both are the manufacturer's estimate under normal storage, and normal storage does not include a windowsill in May.
Rough working lives
Sunscreen is the one to be strict about, because the failure is invisible and the consequence is not. Filters degrade, formulas separate, and a bottle that has spent a summer in a car or a beach bag has had a harder life than the date implies. If a sunscreen has changed colour, texture or smell, or if you cannot remember which summer it belongs to, replace it. This is also the product most likely to be old, since a bottle used in the amount actually required should not survive long enough to expire.
Vitamin C in water-based form is the most visibly unstable common active. It oxidises, and the tell is the colour: clear or pale straw turning yellow, then amber, then brown. Brown means it has largely gone, and why vitamin C serum turns brown explains the chemistry and which forms resist it.
Anything in a jar you dip fingers into has a shorter effective life than the same formula in a pump, because every use introduces bacteria and air. Products in opaque, airless pumps last longest and protect light-sensitive actives best.
Sheet masks are usually fine for a long time sealed and are finished the moment they are opened, since the sachet is single use by design.
Anything oil-heavy can go rancid, and rancidity announces itself: a stale, crayon-like smell that was not there before.
The signs worth acting on
A change in smell is the most reliable one. A separated formula that will not come back together with shaking. A colour shift, particularly darkening. A change in texture, thickening, thinning or grittiness. Anything visibly growing. And in use, stinging or irritation from a product that never used to sting, which sometimes means the formula has changed and sometimes means your skin has, as the purging and reaction post distinguishes.
Products made without strong preservative systems, including anything homemade, anything water-based sold as preservative-free, and freshly mixed clay or herb packs, have very short lives and belong in the same mental category as food rather than cosmetics. That applies to the water-based home preparations covered in rice water and rose water too.
Making them last their stated life
Keep products out of direct sun and away from heat. A bathroom shelf that gets hot and steamy twice a day is the worst common storage spot, and a bedroom drawer is better for anything with a light-sensitive active.
Close things properly, use a spatula rather than fingers for jars, and do not decant into a different container, since the packaging is frequently doing preservation work that a pretty glass bottle will not.
Write the opening date on the bottom in marker. It takes a second and it removes the guessing entirely, which is the only real problem here.
Finally, buying fewer products and finishing them is more effective than any storage advice. A routine of three products used daily, of the kind the routine page lays out, never reaches the age where any of this becomes a question.
