Saying two people are three years apart is usually enough, but sometimes you need the exact gap, in years, months, and days. The method is the same borrow method used for exact ages, applied to two birth dates instead of one birth date and today.
How it works
Subtract the earlier birth date from the later one, working from days up to years. If the day subtraction goes negative, borrow a month and add the days in the previous month. If the month subtraction goes negative, borrow a year and add 12 months. The result is the exact gap. The order does not matter for the size of the gap, but always subtract the earlier date from the later date so you never get a negative answer.
Step by step
- Write both birth dates as day, month, year, with the later date first.
- Subtract the days. If negative, borrow one month and add the days in the previous month.
- Subtract the months, after borrowing. If negative, borrow one year and add 12.
- Subtract the years, after borrowing.
- Read the result as years, months, and days.
- Sanity-check: add the gap to the earlier birth date and confirm you reach the later one.
Worked example
Person A was born 5 June 1988 and person B on 20 November 1995. Days: 20 – 5 = 15. Months: 11 – 6 = 5. Years: 1995 – 1988 = 7. The gap is 7 years, 5 months, 15 days. A borrowing case: A born 22 September 1990, B born 10 February 1998. Days: 10 – 22 = -12, borrow January (31 days) to get 19 days, months drop by one. Months: 2 – 1 – 9 = -8, borrow a year to get 4 months. Years: 1998 – 1 – 1990 = 7. The gap is 7 years, 4 months, 19 days.
Common mistakes
- Subtracting whole years only and ignoring the months, which can be off by almost a year.
- Borrowing 30 days regardless of the month, which shifts the day count.
- Subtracting the later date from the earlier date and reporting a negative gap.
- Forgetting that the gap in whole years changes on the earlier-born person birthday each year.
Try it with the free tool
You can follow every step above by hand, but the free tool below does the same calculation instantly. Try your own numbers in it, then check one result by hand so you trust the method as well as the answer. Open the full tool here: open the tool.
Checklist before you finish
- Write down every input number first, with its unit, before you calculate anything.
- Work through the steps in order, and do not skip the unit conversions.
- Check the answer against the worked example above: is it the right size, and does the direction make sense?
- Round only at the end, and keep the unrounded figure for any follow-up calculation.
Practice with your own numbers
Pick three cases of your own and work each one by hand. First, try a simple round-number case you can check mentally. Second, try a realistic case with awkward numbers, the kind you will actually meet. Third, try an edge case, such as a very small or very large input, and confirm the answer still behaves sensibly. If any case gives a surprising result, re-check the units and the formula step by step before you trust it.
If your answer looks wrong
Most errors come from a small set of causes. Check that you used the same units throughout, that you divided where the formula divides and multiplied where it multiplies, and that you applied any percentage as a decimal. Check the order of operations: powers and brackets come before multiplication and division, which come before addition and subtraction. Finally, test the method on the worked example in this guide. If your method reproduces the example answer, the method is right and the inputs deserve a second look.
Quick tips
For a quick whole-year gap, subtract the years, then subtract one if the later-born person has not yet had a birthday in the year you are comparing. Keep both full birth dates, not just years, whenever the exact gap matters.