Birth and Death Date Calculators

Online Birth and Death Date Calculators

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 exact calendar month method 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 exact calendar month method and provides 30 day month method (8870 method), 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 exact calendar month method or 30 day month method (8870 method)

Calculating Birth Dates

We all know that having birth dates helps us to complete the information for our ancestors and birth dates many times are much harder to find the further back you go. But many times we at least have the date of death with the age listed in Years-months-days.

This information can be found on

death records old cemetery stones

church record possibly Obituaries

Calculating Dates Using Online Calculators

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

• Exact calendar month method
• 30 day month method (also known as 8870 method)

This can be explained with this example:

• Death date: 13 Mar 1870
• Age at death: 5 years 10 months, 13 days,
• Two answers can be given:

• Exact calendar month method: 28 Apr 1864,
• 30 day month method: 30 Apr 1864 (matches 8870 method)

Which Method Should be Used?

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. This 30 day month method would have been the easiest way for the individual to calculate the date for the tombstone inscription.

Tips

• 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.)"
• 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."
• 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.

How to calculate by hand - Estimated date

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

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