Birth and Death Date Calculators

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Online Birth and Death Date Calculators[edit | edit source]

Links Website Description Method
Age Calculator Rootsweb Used to calculate birthdate using date of death and age; calculate age of death with birthdate and death date; calculate death with only the year of birth and death. Includes the calendar month method (subtracts days first) or the 30 day month method (8870 method)
Ancestor Search Age Calculator searchforancestors.com Used to determine age from birth date and death date
Birth Date Calculator longislandgenealogy.com Used to determine birthdate from age at death Uses 30 day month method (8870 method)
Date Calculator Timeanddate.com Calculates birthdate from age at death and age at death from birth and death date Instructions Uses calendar month method. Shows results from subtracting years first AND shows results subtracting days first, if different.
Tombstone Birthday Calculator searchforancestors.com This calculator allows Julian calendar or Gregorian dates to help you calculate birth date from age at date of death. Has options to use calendar month method (subtracts days first) or 30 day month method (8870 method)

Calculating Birth Dates[edit | edit source]

Sometimes a birthdate is not know and the date of death with the age listed in Years-months-days is provided. This requires a calculator to be used to determine the birthdate. There are several different kinds of calculators to use in determining a person's birth or death date. The method used by the calculator will give you different results.

Age of death can be found in:

  • death records
  • old cemetery stones
  • church records
  • some obituaries

Ways for Calculating Dates[edit | edit source]

There are different ways to calculate a date from an age at death:

  • Calendar month method
  • In addition, the calendar month method can be done two ways:
  • Subtract days first, then months and years
  • Subtract years first, then months and days
  • 30 day month method (also known as 8870 method)


This can be explained with this example:

  • Death date: 1 Apr 1870
  • Age at death: 0 years 1 month, 4 days
  • Three answers can be given:
  • Calendar month method:
  • Subtract days first, then months and years: 28 Feb 1870
  • Subtract years first, then months and days: 25 Feb 1870
  • 30 day month method: 27 Feb 1870 (matches 8870 method)

Which Method Should be Used?[edit | edit source]

Although, the calendar month method may take into account the correct number days of the month, it is a common belief that the 30-day-month method or the 8870 hand method used for years before calculators is the method most often used. [1] This 30 day month method would have been the easiest way for the individual to calculate the date for the tombstone inscription.

Tips[edit | edit source]

  • Be aware that some of these websites have other types of calculators with different functions, explanations and tips, with various kinds of information.
  • Some calculators may require you to put a 0 in a box instead of leaving a box blank, also using a 0 in front of a single digit month may or may not be required.
  • Due to possible errors or different methodology used in Calculated dates from original calculations. (found on the tombstone, obituary or death certificates) , please make a note in your records that the date was calculated..
  • Calculators can be used to approximate a birth year from: 1) census data 2) birth date from wedding date and age.
  • Use either the Julian or Gregorian Calendar. Do not used the two together.
  • There is what is called a 30-day-month/or **8870 method which assumes that all months have 30 days. As searchancestors.com says: "...many of the headstones and obituary ages at death appear to have been computed using this method. It's an approximation which agrees with the exact calendar month method about 2/3 of the time.)"[2]
  • searchforancestors.com: "Because of the change from the Julian to the Gregorian Calendar, the calculator may not work for anyone born between Sept. 2, 1752 and Sept. 14, 1752 or when days were lost at the calendar changeover."[3]
  • The calendar was changed from the Julian to Gregorian calendar at some point in a country's history. Each country had a different date. Be sure to do the calculation with the correct calendar. Use this resource: change from Julian to Gregorian calendar Dates by country.[4]

How to calculate by hand - Estimated date[edit | edit source]

There is what is called a 30-day-month/or **8870 method which assumes that all months have 30 days, many of the headstones and obituary ages at death appear to have been computed using this method. It's an approximation which agrees with the exact calendar month method about 2/3 of the time.

When the online calculator is not available, use the **8870 formula or the 30 day month length [5]

For example: If a person died May 6, 1889 at age 71 years, 7 months and 9 days:


18890506 Year, day, month of death (yyyymmdd) 1889--05--06

-710709 Subtract age at death (yymmdd) 71yr-07mo-09dd 

_________ 18179797

   -Subtract constant 8870

________ 18170927 Born 27 Sept. 1817 (yyyymmdd) 1817yr-09mo-27dd

This person was born on September 27, 1817

References[edit | edit source]

  1. searchforancestors.com Tombstone Birthday Calculator, http://www.searchforancestors.com/utility/birthday.html, accessed 25 June 2021. Quote: "While you may think the calendar month method may be more reliable, be aware that many of the headstone and obituary ages at death appear to have been computed using the 30-day-month method. (The 30-day-month method assumes that all months have 30 days. It's an approximation which agrees with the calendar month method about 2/3 of the time.)"
  2. Amy Larner Giroux, Skillbuilding: Date Calculations https://bcgcertification.org/skillbuilding-date-calculations/ accessed 30 June 2021;Ancestor search
  3. Ancestor search
  4. Ancestor search
  5. Search for ancestors