Birth and Death Date Calculators

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

Name Website Description
Date Calculator: Add to or Subtract From a Date Timeanddate.com Can be used for calculating birth from tombstones, death records when exact age is given in yr/mo//dd also a Duration Between Two Dates age calculator
Age Calculator Rootsweb Has 4 various calculators to determine birth date and age including the **8870 method
Ancestor Search Age Calculator searchforancestors.com Used when you want to determine the date of death from a birth date and exact age
Tombstone Birthday Calculator searchforancestors.com This calculator allows Julian calendar or Gregorian dates to help you calculate birth date when the tombstone only gives the date of death and the exact age. This calendar also allows for a 30 day month or **(8870) method
Birth Date Calculator longislandgenealogy.com Used to help you calculate birth date when the tombstone only gives the date of death and the exact age

Calculating Dates[edit | edit source]

Having exact birth and death dates for our ancestors helps us confirm our finds, as well as better identify them in other records. However, finding precise dates can be much more difficult as we research further back in time. We may only have our ancestor’s age (sometimes in the format of years/months/days) at time of death. Or, we may wish to know how old an ancestor would be from their birth date. Using a date calculator can help us with the addition and subtraction necessary to discover our a more precise date. The date calculators listed above can help you determine that information.

Records that may give an exact age at death and a death date can be found on:

  • Death records
  • Grave markers (typically older markers)
  • Church records
  • Obituaries

Tips for Using Date Calculators[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. So check out the rest of the page. (The following examples are what you might find for tips)
  • 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..
  • The calculator could also be used to calculate birth year from census data or birth date from wedding date and age.
  • When calculating Dates using the Julian or Gregorian Calendar you can't use the two together.
  • 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.) [1]
  • 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.[2]
  • Be aware the calendar was changed from Julian to Gregorian in which ever country for which you are performing the calculation. See the change from Julian to Gregorian calendar Dates by country.[3]

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 [4]

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]