Find what year someone was born from their age, or an exact birth date from an age in years, months and days, as of today or any other date.
Related Calculators
- Date Calculator
- When Is My Half Birthday Calculator
- Graduation Day Calculator
- Marriage Date Calculator
- All Everyday Life and Education Calculators
Reverse Age Formula
The calculator finds every birth date matching either a whole age or an age in completed years, additional months and days on a reference date.
Whole-age range
anniversary(B, age) ≤ ReferenceDate < anniversary(B, age + 1)
Here B is a candidate birth date. An anniversary that would fall on an invalid day clamps to that month’s last day. In this convention, February 29 anniversaries fall on February 28 in non-leap years. This is a stated arithmetic convention, not a legal age rule.
Years, months and days
Take the greatest completed years, then the greatest additional months, measuring combined years and months from the original birthday and clamping the destination day once. The remaining days are actual elapsed calendar days. The calculator checks candidate dates against that forward rule rather than assuming reverse subtraction identifies a unique birthday.
Blank additional months and days mean zero. The reference date defaults to your device’s current local date and can be changed. Calculations use date-only Gregorian arithmetic, without daylight-saving offsets. Supported years are 1–9999.
Reference Tables
These illustrative birth-year group labels are conventions, not universal generation boundaries. The first table shows ages reached by December 31, 2025; the second gives approximate duration equivalents, not exact calendar ages.
| Generation | Birth Years | Age by Dec.31,2025 |
|---|---|---|
| Gen Alpha | 2013–2024 | 1–12 |
| Gen Z | 1997–2012 | 13–28 |
| Millennials | 1981–1996 | 29–44 |
| Gen X | 1965–1980 | 45–60 |
| Boomers | 1946–1964 | 61–79 |
| Silent | 1928–1945 | 80–97 |
| Age (years) | Approx. Months | Approx. Weeks | Approx. Days |
|---|---|---|---|
| 10 | 120 | 521 | 3,652 |
| 21 | 252 | 1,096 | 7,670 |
| 35 | 420 | 1,826 | 12,784 |
| 50 | 600 | 2,609 | 18,262 |
| 65 | 780 | 3,391 | 23,741 |
Worked Examples
Example 1 — Whole age. Someone age 35 on 2026-10-22 could have been born from 1990-10-23 through 1991-10-22, inclusive.
Example 2 — Calendar age. An age of 35 years, 2 months and 10 days on 2026-10-22 matches 1991-08-12 under the stated rule.
Example 3 — Month-end ambiguity. An age of 0 years, 1 month and 0 days on 2025-02-28 matches January 28, 29, 30 or 31, 2025. Each date shifts to February 28 under month-end clamping.
FAQ
Why are several birth dates possible? A whole age leaves the birthday unknown. Even years, months and days can have several matching dates when month-end clamping maps different birthdays to one anniversary. All matching dates appear in calculation details.
Does it handle leap years? Yes. For example, age 1 on 2025-02-28 includes births on 2024-02-29 under the stated February 28 anniversary convention. Different legal or administrative rules may use another convention.
Can the reference date be in the future? Yes. Enter the age expected on that date. Results are arithmetic possibilities, not verification of anyone’s age or identity.
