Birth and Death Date Calculators
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 Yearsmonthsdays 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:


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 30daymonth 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 30daymonth/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 30daymonth/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)
18890506
710709 Subtract age at death (yymmdd) 71yr07mo09dd
_________ 18179797
Subtract constant 8870
________ 18170927 Born 27 Sept. 1817 (yyyymmdd) 1817yr09mo27dd
This person was born on September 27, 1817
References[edit  edit source]
 ↑ 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 30daymonth method. (The 30daymonth 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.)"
 ↑ Amy Larner Giroux, Skillbuilding: Date Calculations https://bcgcertification.org/skillbuildingdatecalculations/ accessed 30 June 2021;Ancestor search
 ↑ Ancestor search
 ↑ Ancestor search
 ↑ Search for ancestors