Shelf life added to a start date
The expiry date is the start date plus the shelf life. To count how much time is left, the shelf life is converted to days, added on, and the gap to today is measured.
Weeks count as 7 days and years as 365. Months are treated as about 30 days, so a date built from a month count can land a day or two off a strict calendar month. For a firm deadline, prefer a shelf life stated in days.
The label word decides what the date means
Two products can carry the same date and mean very different things. Quality dates are advisory; safety dates are not.
| Label | What it means | After it passes |
|---|---|---|
| Best before | Quality guarantee | Usually still safe; may lose flavor or texture |
| Use by | Safety deadline | Do not eat |
| Sell by | Stock date for the retailer | Not aimed at you; buy before it for freshness |
Typical unopened shelf life
| Product | Shelf life | Stored |
|---|---|---|
| Milk | 7–10 days | Refrigerated |
| Yogurt | 3–4 weeks | Refrigerated |
| Eggs | 4–5 weeks | Refrigerated |
| Butter | 4–6 months | Refrigerated |
| Flour | 6–8 months | Cool, dry pantry |
| Hard cheese | 6–12 months | Refrigerated, sealed |
| Canned goods | 2–5 years | Cool, dry pantry |
These assume proper storage. Heat, damp or light shorten real shelf life below the printed date. Freezing pauses spoilage but does not reset the date once thawed.
Common questions
How is the expiry date worked out?
Add the shelf life to the start date. The tool converts the shelf life to days first (weeks are 7 days, months about 30, years 365), adds them to the packaging or production date, then counts the days between that result and today to show how long is left.
Is "best before" the same as "use by"?
No. Best before is about quality, so the food is usually still safe after it if stored well, it just may lose flavor or texture. Use by is a safety deadline; do not eat the product after it. Sell by is for the shop, not you.
Does the expiry date still hold after I open the package?
No. The printed date assumes an unopened package. Once opened, the safe window is usually much shorter, sometimes only a few days, so follow any separate "once opened" guidance on the label.
