Date of Birth Calculator
Enter an age in years, months and days as of a given date, and get the exact birth date it implies — plus the wider range when only whole years are known.
Stated ages are usually truncated, not rounded — “30 years old” means the birthday already happened this year, anywhere from today back to just after last year's. The exact date on the left assumes the months and days you entered are precise; the range on the right is the honest answer when they aren't.
Frequently Asked Questions
How do you calculate birth date from age?
By reversing the same calendar arithmetic an age calculator uses: subtract the stated days, then the stated months, then the stated years, in that order, from the "as of" date. If the age you were given is exact to the day, this returns a single exact birth date.
Why does the tool also show a range instead of just one date?
Because a stated age like "I'm 30" is almost always a whole number, truncated rather than rounded — it only tells you the birthday already happened this year. Without a month or day, the true birth date could be anywhere in a roughly year-wide window; the range shows exactly how wide that window is for the years you entered.
What does "as of" mean here?
The date the stated age was true. If someone told you their age today, leave it as today; if you're reconstructing a birth date from an age recorded on an old document, set "as of" to that document's date instead.
What if the person was born on February 29?
The reversal cannot recover a February 29 birthday exactly. An age counted in years, months and days treats February 28 as the anniversary in non-leap years, so reversing that age returns February 28. If a leap-day birthday is possible, check both February 28 and 29 against the age you were given.
Is this the same as looking up someone's actual birth date?
No — it's the birth date (or range of birth dates) that's mathematically consistent with the age you were given. If that age was itself approximate, the result inherits that uncertainty.