How to Calculate the Age Difference Between Two People

“How much older is Person A than Person B?” Sounds simple — but doing it precisely (in years, months, and days) requires calendar-aware arithmetic.

The calculation method

1. Identify which date is earlier (the older person)
2. Subtract: later year − earlier year = initial years
3. Adjust months: if the later person’s birth month is before the earlier person’s, subtract one year and add 12 months
4. Adjust days: if the later day is before the earlier day, subtract one month and add the previous month’s day count

Worked example

Person A: March 15, 1990
Person B: August 7, 1993

Years: 1993 − 1990 = 3
Months: 8 − 3 = 5 (no borrowing needed)
Days: 7 − 15 = -8 → borrow 1 month
  Months: 5 − 1 = 4
  Days: 7 + 31 (July has 31 days) − 15 = 23

Result: 3 years, 4 months, 23 days
Total days: 1,241 days

Person A is older by 3 years, 4 months, and 23 days.

Total days vs calendar difference

• Total days = simple millisecond subtraction ÷ 86400000. Unambiguous.
• Years/months/days = more relatable but depends on how you handle month boundaries

Both are valid — the “total days” method avoids calendar edge cases entirely.

When age differences are relevant

• Relationships: Understanding age gaps in partnerships
• Siblings: “My brother is exactly 2 years and 4 months older”
• Eligibility: Age-dependent rules (school cutoffs, retirement)
• Historical: Comparing ages of public figures at specific events
• Legal: Age of consent and majority vary by jurisdiction

Fun facts about age gaps

• The average sibling age gap worldwide is 2–3 years
• The average partner age gap in most countries is 2–4 years
• The “half your age plus 7” rule is a social norm, not a law
• Twins born on different days (around midnight) technically have a 1-day age difference

Calculate exact age differences with the OurDailyCalc age difference calculator.

TL;DR

• Subtract dates using year/month/day borrowing (like long subtraction)
• Total days = simpler (just millisecond math)
• A proper calculator handles leap years and variable month lengths
• Both approaches give useful but slightly different perspectives