Expiration Date Calculator (best-before / use-by date from production date)

Understanding the expiration date calculator — concept and principle

This calculator finds the expiry date of a product from its production date and shelf life in days, months or years, and shows how many days are left and how much of the shelf life has been used. It is handy for food, cosmetics, medicines and supplies that print only a production date and a shelf-life period.

How it is calculated

• Days: expiry = production date + number of days.
• Months or years: the months are added on the calendar. If the target month is shorter, the date moves to that month's last day (31 Aug + 6 months = 28 or 29 Feb).
• Days left = expiry − today.
• Shelf life in days = expiry − production date.
• Share used = (today − production) ÷ (expiry − production) × 100.

The principle

Calendar months have 28 to 31 days, so "six months" is not a fixed number of days. Adding months on the calendar keeps the same day of the month where possible, which is how most labels and contracts count. When that day does not exist in the target month, the last day of the month is used. The share used tells you at a glance how far through its life a product is, which helps with rotating stock. In Korea, most food has carried a use-by date (소비기한), the period it is safe to eat if stored as directed, instead of a sell-by date (유통기한) since 1 January 2023; refrigerated milk follows from 2031.

Worked example

Default inputs: produced Oct 1, 2026, shelf life 6 months, viewed on Nov 10, 2026.

1. Oct 1, 2026 + 6 months = Thu, Apr 1, 2027.
2. Shelf life in days = Oct 1, 2026 to Apr 1, 2027 = 182.
3. Days left = Nov 10, 2026 to Apr 1, 2027 = 142, so the status is still within the date.
4. Share used = 40 ÷ 182 = 22%.

How to read the result

A date in the future means the product is within the period the maker states, if it has been stored as directed. Storage matters as much as the date: heat, sunlight and opening the pack can shorten real shelf life. If the label shows a different expiry, the label wins, because makers may count the last day differently.

Common mistakes

• Counting 6 months as 180 days.
• Using the purchase date instead of the production date.
• Ignoring "use within X days after opening" instructions.
• Treating a best-before date as a safety deadline, or a use-by date as a quality guide.

Frequently asked questions

How do I calculate an expiry date from a manufacture date?

Add the shelf life on the calendar; for months, keep the same day of the month, or use the month's last day if that day does not exist.

What is the difference between 소비기한 and 유통기한 in Korea?

소비기한 is how long food is safe to eat if stored properly; 유통기한 was the period for selling it. Korea switched to 소비기한 in 2023.

Is 6 months the same as 180 days?

No. Six calendar months are usually 181 to 184 days.

Sources: Korea Ministry of Food and Drug Safety (식품의약품안전처), 소비기한 표시제 Q&A (korea.kr) · ISO 8601-1:2019 (Gregorian calendar dates)