Birth Date Calculator by Age
Find a birth date from an age measured on a selected date. Enter completed years for the full possible birth-date range, or add months and days to recover every calendar date consistent with that exact age.
How the birth date calculation works
A completed age in years does not identify one birthday. If a person is Y years old on a selected date, the calculator tests the surrounding calendar dates and returns every date whose completed-year age equals Y. The earliest and latest matches form the possible birth-date range.
For an exact age, each candidate date is checked with calendar arithmetic:
Exact age = completed years + remaining completed months + remaining calendar days.
The tool does not treat a month as a fixed number of days. Birthdays on the 29th, 30th or 31st are clamped to the final valid day when a comparison month is shorter. Because of that rule, an exact age can occasionally match two birth dates; the calculator lists both instead of silently choosing one.
Checked examples
| Age on date | Age entered | Birth date result |
|---|---|---|
| September 20, 2026 | 40 completed years | September 21, 1985 through September 20, 1986 |
| September 20, 2026 | 40 years, 0 months, 0 days | September 20, 1986 |
| September 20, 2026 | 26 years, 8 months, 5 days | January 15, 2000 |
| February 28, 2025 | 1 year, 0 months, 0 days | February 28 or February 29, 2024 under month-end clamping |
| March 1, 2025 | 1 year, 0 months, 1 day | February 28 or February 29, 2024 under month-end clamping |
| September 20, 2026 | 0 years, 0 months, 0 days | September 20, 2026 |
How to use the calculator
- Choose whether you know only completed years or the exact years, months and days.
- Enter the date on which the age was measured.
- Enter the age components.
- Select Calculate birth date.
- Review the possible range, exact date or multiple valid month-end dates.
Limits and interpretation
- Calendar dates from 1600 through 2400 are supported, with ages from 0 through 150 years.
- A years-only result is a range, not an exact birthday.
- The calculation uses calendar dates without birth time or time zone.
- Month-end clamping is an explicit arithmetic convention. Legal, insurance, school, sports or government age rules can differ, especially for February 29 births.
- The tool cannot recover a date from a rounded, estimated or incorrectly reported age.
- It does not identify or verify a person.
Frequently asked questions
Can age alone reveal an exact birth date?
No. Completed years normally allow about one year of possible birth dates. Add completed months and days to narrow the result.
Why can an exact age produce two dates?
Dates near the end of February can reach the same clamped anniversary. For example, February 28 and February 29, 2024 both evaluate as exactly one year old on February 28, 2025 under this method.
Does the calculator include leap years?
Yes. It evaluates actual Gregorian calendar dates and includes February 29 in leap years.
Is this suitable for legal age decisions?
Use it as calendar arithmetic only. Confirm the jurisdiction’s rule for eligibility, contracts, benefits, enrollment or other legal decisions.
Related calculators
Use the Age Calculator for the forward calculation from birth date to exact age, the Date Calculator for general date differences, or the Graduation Date Calculator for an academic completion projection.
If both birth dates are known, the Age Difference Calculator compares them directly and also shows each person’s age on a selected date.