Genealogy Age & Date Calculator
Use calculations as clues, not as substitutes for records. A date produced by a calculator is a calculated date. It does not become a documented birth or death date until supported by appropriate evidence.
Tombstone Birthday Calculator
Calculate a birth date from a known date of death and an age stated in years, months, and days.
8870 Method
Try the historical genealogy shortcut used to calculate a birth date from a death date and age at death.
About this method: The 8870 formula is a historical arithmetic shortcut. The death date is written as YYYYMMDD, the age is written as YYMMDD, the age is subtracted, and then the constant 8870 is subtracted.
It can occasionally produce values that require old-style arithmetic adjustment. For research, compare the result with the standard Tombstone Birthday Calculator and preserve the original wording of the source.
Age at Death Calculator
Calculate the elapsed age in years, months, and days between a birth date and a death date.
Approximate Age at Death
Use only the birth year and death year when complete dates are not known.
With years alone, we do not know whether the person had already had a birthday in the year of death. The most defensible result is usually a two-age range rather than one exact age.
Genealogy Notes: What Does the Result Prove?
Calculated Is Not Documented
If a birth date is derived from a death date and stated age, label it as calculated unless an independent birth or baptism record confirms it.
Preserve the Source Wording
Record whether the source says “aged 71 years, 7 months, 9 days,” “age 71,” “in his 72nd year,” or something else. Those statements are not interchangeable.
Watch Historical Calendars
These tools use Gregorian calendar arithmetic. For early records, investigate Old Style/New Style dating and local calendar practice before treating a calculated date as exact.
Research standard: Let the surviving record define the level of certainty. A calculated date can be a very useful research target, but precision in the answer cannot make the underlying evidence more precise than it actually is.
Calculate the Date. Then Find the Record.
Genealogy calculators are excellent tools for building search windows, checking timelines, and interpreting tombstone inscriptions. Keep the calculation attached to its source, preserve uncertainty, and continue looking for records that can confirm—or correct—the result.