Cosmetics carry a period-after-opening (PAO) symbol – an open jar with “6M” or “12M” – that tells you how many months they stay good once opened. Medicines, sealants, adhesives and food have shelf lives from manufacture. Knowing the exact date avoids using products that may have lost effectiveness.
How to use the Expiration Date Calculator
- Production or opening date
- Shelf life
- Unit
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the production or opening date and the shelf life with its unit.
Tips
- Use the figures printed on your product or its manual when you have them.
- Round quantities up so you are never short.
- Measure twice before you buy anything cut to size.
- Everything runs in your browser – try different values to compare options.
Common mistakes to avoid
- Mixing units (inches and centimetres, mAh and Ah).
- Ignoring efficiency losses and safety margins.
- Treating rules of thumb as exact.
Inputs explained
| Field | What to enter |
|---|---|
| Production or opening date | A date |
| Shelf life | A number |
| Unit | Choose one: Days, Weeks, Months, Years |
What each result means
- Day of the week = weekday([Expiration date])
- Days left from today = [Expiration date] – today()
Formula
Expiration date = start date + shelf life (months and years added by calendar date)Days left = expiration date − todayWorked example
Inputs — Production or opening date: 2026-08-26; Shelf life: 12; Unit: Months.
Results — Expiration date = Thu, August 26, 2027; Day of the week = Thursday; Days left from today = 305; Status = In date.
Frequently asked questions
What does 12M on a cosmetic mean?
Use within 12 months of opening (the period-after-opening symbol).
Is food unsafe after the best-before date?
"Best before" is about quality; "use by" is about safety – don't eat food after its use-by date.
How long do opened cosmetics last?
Mascara 3–6 months, foundation 6–12 months, lipstick 12–24 months.
What formula does the Expiration Date Calculator use?
Expiration date = start date + shelf life (months and years added by calendar date).