What this calculator does
Find out exactly how far apart two people are in age, in years, months and days, and who is older. The gap never changes, because it is the time between the two birth dates. Getting it exact takes some care, since months have different lengths and leap years add an extra day. The calculator handles all of that for you.
It also answers a classic puzzle: when was (or will be) one person exactly twice the other's age? That happens once, on the day the younger person's age equals the age gap. The calculator finds that date and says whether it is in the past or the future.
Where it's used
Why this math matters outside the classroom:
- Siblings. Brothers born on June 1, 2015 and September 1, 2018 are 3 years and 3 months apart.
- Twice the age. When two people are 6 years apart, the older one is exactly twice the younger one's age when they are 12 and 6.
- Family history. Genealogists compare birth dates in records to work out how far apart relatives were in age.
- Couples. Partners find the exact gap between their birthdays in years, months and days.
How to use it
- Enter the first person's date of birth.
- Enter the second person's date of birth. The order doesn't matter; the calculator works out who is older.
- Leave today's date as it is, or change it to see ages as of another day.
- Read the age gap, the gap in days and decimal years, both current ages and the twice-the-age date.
What the inputs mean
- Person 1 date of birth
- The date of birth of the first person. Birth dates stay in your browser; this page has no shareable link with the values.
- Person 2 date of birth
- The date of birth of the second person. It can be earlier or later than the first.
- Today's date
- Used to show each person's current age and to decide whether the twice-the-age date is in the past or the future. Birth dates can't be after this date.
Formula and method
age gap = later birth date − earlier birth date (whole years and months, then days) gap in days = day number(later) − day number(earlier) older age = 2 × younger age ⇔ date = younger's birth date + gap in days decimal years = gap in days ÷ 365.2425
The gap in years, months and days is counted forward from the earlier birth date: as many whole months as fit, then the remaining days. When a month anniversary falls on a day the month doesn't have (such as the 31st), it moves to the month's last day.
For the twice-the-age date, let g stand for the gap in days and t for any date. On date t, the older person is (t − older birth) days old and the younger is (t − younger birth) days old. The older age is double the younger when t − younger birth = g, so the date is the younger person's birth date plus g days. On that day the younger person is g days old and the older person 2g days.
Worked example
Person 1 was born on April 12, 1988, and person 2 on September 30, 1993. Today is October 18, 2026.
- Person 1 was born first, so person 1 is older.
- Counting from Apr 12, 1988: 5 whole years to Apr 12, 1993, 5 more months to Sep 12, 1993, then 18 days to Sep 30, 1993.
- In days: Sep 30, 1993 − Apr 12, 1988 = 1,997 days, or 1,997 ÷ 365.2425 = 5.47 years.
- Twice the age: Sep 30, 1993 + 1,997 days = Mar 20, 1999. Person 1 was then 3,994 days old, twice person 2's 1,997 days.
Result: Person 1 is older by 5 years, 5 months and 18 days, and was exactly twice person 2's age on March 20, 1999. Today they are 38 years, 6 months, 6 days and 33 years, 0 months, 18 days old.
Understanding the result
The age gap in years, months and days stays the same for life. What changes is the ratio between the ages: it shrinks toward 1 as both people get older, which is why the twice-the-age moment happens only once.
The twice-the-age date is exact in days. Expressed in years and months, the two ages may not look like a perfect double because months are not all the same length.
Assumptions and limitations
- Both dates use the Gregorian calendar, the standard calendar used in most of the world today.
- Ages are counted in whole calendar days, ignoring the time of birth and time zones.
- Birth dates after the chosen today's date are not accepted.
- Twins and others born on the same day have no gap and no twice-the-age date.
- The decimal-year figure uses an average year length, so it is approximate for short gaps.
Frequently asked questions
Why doesn't the gap in days divided by 365 match the years?
Leap years add a day every four years or so, and the years-months-days figure follows the calendar. 1,997 days is 5 years, 5 months and 18 days in this example, but 1,997 ÷ 365 would give 5.47 with a small error that grows over longer gaps.
Does the order I enter the people in matter?
No. The calculator compares the dates and labels whoever was born first as older. The gap is the same either way.
Will one person ever be twice the other's age again?
No. After that date the younger person's age keeps growing faster in relative terms, so the ratio of the ages keeps falling toward 1. It reaches exactly 2 only once.
How is someone born on February 29 handled?
In years without February 29, month anniversaries that would fall on the 29th move to February 28. The day count is unaffected because it counts actual calendar days.
How this calculator is checked
Method: Calendar difference on the Gregorian calendar (whole months, then days, with end-of-month clamping); twice-age date solved from t − b_older = 2 (t − b_younger). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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