There is a sunscreen in your bathroom drawer that you bought last May. One holiday, a few months of walking to the station, and then autumn arrived and it went in behind the hair ties. It is September again and the bottle still has weight when you shake it.

Almost everyone does the same thing next. Turn it over, hunt for a date. Right instinct, wrong number. The date stamped on a cosmetic is normally a shelf life counted from the manufacturing date, and it describes a bottle nobody ever opened. Yours has been open since last May.

What you actually want to know is what happened to the bottle in between. That part is not printed anywhere, but you can work most of it out yourself in about a minute.

Sunscreen ages differently from the rest of your shelf

A sunscreen is a set of UV filters carried in an emulsion, and both of those dislike the same three things: heat, light and air. Nearly every sun product tells you on the back to keep it in a cool dry place out of direct sunlight. On a sun product that sentence is doing real work rather than covering the manufacturer.

Compare where a sunscreen lives with where your toner lives. The toner has never left a shelf indoors, room temperature, in the dark. The sunscreen went in the beach bag. It sat in the car door through August. It rode in the outside pocket of a backpack that spent an afternoon lying on hot sand. Two bottles with the same printed date can be in completely different condition, and the print has no way of knowing which one you are holding.

Air counts too. Every opening lets some in, and a wide screw cap lets in more than a pump or a tube. A bottle you opened every single morning for three months has had a different life from one you opened twice and forgot.

What you can check at home, and what you cannot

You cannot measure how much protection is left in there. No home test gives you that, and anyone selling you one is guessing. What you can check is whether the formula is still the formula it was sold as.

1. Find the date and read it as the unopened date. That figure describes the best case, a bottle still sealed in a warehouse.
2. Look for the small open jar drawing with a number and the letter M beside it. That is months after opening, and that clock started the first morning you used it.
3. Be honest about where it lived. A bottle that spent a summer in a car should be treated as older than either number suggests.
4. Squeeze some onto the back of your hand. A watery layer sitting on top, visible grain, a colour that has drifted, a smell that has gone sharp. Any of those means the emulsion has separated, and that is enough on its own to stop.

Then there is the reading nobody enjoys. A sunscreen bottle that survives two summers was not being used the way sun care is meant to be used. Face and neck every morning, then again every two to three hours on the days you are actually outside, empties a small bottle much faster than people expect. A bottle still half full in September is a report on last summer's habit more than on the bottle.

Which is why the size of the next one, and how it feels going on, matter more than they look like they should. A sunscreen you do not enjoy wearing is a sunscreen you will still own next year.

Someone outside this shop talking about expiry dates on sun care. It is their view, we had no part in it, and we are not summarising what they say.

Video: SUNSCREEN & SKIN CARE EXPIRATION DATES| DR DRAY / Dr Dray. Not affiliated with BRAND KR.

Where our sun serum fits into that

We sell the REJURAN Water Capsule Sun Serum, 40mL, SPF50+ PA+++, broad spectrum across UVA and UVB. Origin is South Korea, the distributor is PharmaResearch, and the page lists it as a functional cosmetic for UV protection. The stated shelf life is 36 months from the manufacturing date unopened, with the same cool dry storage instruction described above.

Two details on that list connect to the argument above. First, 40mL. That is a small bottle, and if you are covering face and neck each morning and topping up outdoors, small is the point. You finish it inside a season and the shelf life question never comes up at all. Second, the texture. It is sold as a serum rather than a cream, built around water capsules that burst on application, and the page describes it as absorbing quickly without greasiness, with no pilling or stickiness under makeup. Pilling is the quiet reason most people stop reapplying at lunchtime.

Thirty dollars, all skin types, and it goes on as the final step of skincare.

Who this suits

Someone who kept the old bottle because the bigger size felt like better value, and now owns half a bottle in unknown condition. Someone who gave up on midday reapplication because the last one rolled off under foundation. Someone who wants sun care to sit in the routine where a serum sits, instead of feeling like a separate job at the end.

Who should skip it

If you already get through a bottle every six or eight weeks, none of this is your problem and you should keep buying whatever you are buying. If you spend long days outside and want the highest UVA rating printed on a label, note that this one is PA+++ rather than PA++++. And if your skin prefers something with more cushion under a heavy base, a cream or a stick will suit you better than a serum.

How to use it

1. Apply a moderate amount as the final step of your skincare.
2. Spread it evenly across face and neck.
3. Reapply every two to three hours when you are exposed to sunlight.
4. Keep it away from your eyes. Stop using it and speak to a specialist if your skin reacts.
5. Store it somewhere cool and dry, out of direct sun. Not the car door.

One thing it will not do for you

It cannot tell you how it has been stored. No sunscreen can. The 36 month figure on the product page describes an unopened bottle sitting somewhere sensible, and from the day it starts travelling in your bag that number stops being the whole story. Forty millilitres used the way the instructions describe, face and neck daily plus reapplication on outdoor days, is also not a supply that stretches across an entire summer. We would rather say that now than have you discover it in July. Buy it as something you finish, not something you keep.

If that trade sounds right, the page is here: REJURAN Water Capsule Sun Serum 40mL. And whatever you decide about it, go and look at the bottle in the drawer today, rather than on the first hot morning next year.