Calculate elapsed days, remaining days and percentage remaining with a moving calendar timeline. The example also explains why calendar arithmetic cannot establish product safety.
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The calculation
Total interval = end date − start date; remaining % = remaining days ÷ total days × 100
Worked example
April 1 to May 1 is 30 elapsed days. On April 10, 9 days have passed and 21 remain. The remaining share is 21 ÷ 30 × 100 = 70%. Half remains on April 16.
Count elapsed intervals between dates rather than counting both endpoint labels. Use actual calendar dates, and assess product instructions, storage and opening separately.
Video chapters
- 0:00 — A date interval, not a safety test
- 0:13 — Count the gaps between dates
- 0:30 — April 1 → April 10: 9 days
- 0:46 — 21 ÷ 30 × 100 = 70%
- 1:02 — Half remaining means halfway through
- 1:18 — Months do not all have 30 days
- 1:34 — Storage and labels need separate checks
- 1:54 — Subtract dates, then calculate the share
Read the full transcript
A product has a start date of April first and an end date of May first. How much of that interval remains on April tenth? We can calculate the calendar time and its percentage. The arithmetic cannot tell us whether a product is safe.
First define the counting convention. From the beginning of April first to the beginning of April second is one elapsed day. Count the gaps between dates, not both endpoint labels. April first to May first is thirty days because April has thirty days.
Move the check date to April tenth. Nine days have elapsed since April first. Subtract those nine days from the full thirty-day interval. That leaves twenty-one days until May first. The two colored parts must still add up to thirty.
To express the remaining time as a percentage, divide remaining days by total days, then multiply by one hundred. Twenty-one divided by thirty equals zero point seven. That is seventy percent remaining. The elapsed part is the other thirty percent.
When will half remain? Half of thirty days is fifteen days. Add fifteen elapsed days to April first and you reach April sixteenth. On that date, fifteen days have passed and fifteen remain. A percentage milestone belongs to this particular interval.
Do not replace every calendar month with thirty days. Months differ in length, and leap years change February. Work with the actual dates. If the end date is earlier than the check date, a negative difference means the stated interval has already ended.
For food, date-label meanings and storage conditions matter. A best-quality date is different from proof of safety, and opening a package can change storage guidance. Use the product instructions and relevant food-safety advice. Do not extend a usable life merely because this calculation shows time remaining.
The method is: define the start and end dates, count elapsed calendar days, subtract to find the remainder, and divide by the total for a percentage. In our example, twenty-one of thirty days remain: seventy percent of the stated interval.