How this calculator works
Given a death date and an age at death, the years and months are stepped back on the calendar and the leftover days are subtracted. The result is checked by measuring the age forward from the candidate birthday to the death date, because month lengths make direct subtraction ambiguous by a day or two near a month end. The weekday and the number of days lived are then reported for the candidate date. A record that gives an age only to the year will be off by up to almost a year, so treat the result as a window rather than a certainty.
Formula
Candidate = death date minus (12 x years + months) calendar months, then minus days. The candidate is accepted when ageParts(candidate, death date) equals the requested years, months and days. days lived = O(death date) - O(candidate).
Worked example
A death on 2026-11-01 at age 80 years gives an estimated birth date of 1946-11-01, a Friday, and 29,220 days lived. A death on 2023-06-15 at 62 years, 2 months and 10 days gives 1961-04-05.
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Frequently asked questions
Why is the birth date only an estimate?
Historic records often give an age at death rounded to the year, and the birthday itself is unknown. Even an exact age leaves open which date in the year was the birthday. Use the nearby table to see how a different age moves the estimate.
What if the record crosses the old and new calendars?
This page uses the proleptic Gregorian calendar. Dates recorded before a jurisdiction adopted the Gregorian calendar may need a separate calendar conversion, and the difference can be ten or eleven days where the Julian calendar was still in use.
How is a February 29 birth handled?
The anniversary is placed on February 28 in common years. A person born on February 29 will therefore appear to have their birthday one day earlier in most years, which is enough to change the estimate by a day around February and March.
Can the death date be before the birth date?
No. An age of zero or more always places the birth date on or before the death date, and a combination that would not is rejected rather than returning a date after the death.