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
So how does one calculate a date to get as close as possible to the date of birth? By using a Date Calculator. **However there are two ways a date calculator may be formulated to calculate such dates. If given certain scenarios this link will calculate both ways. We can't be 100% sure which method was used to calculate the age at death. (check NOTE)
- The way to use it is to put in the date of death. Subtract the number of Years, Months, (weeks if given), Days; then click the calculate button. The answer is under the lower left corner under the colored box.
- If for some reason you have the Date of Birth and have the number of Years/Months/Days when died you would put in the date of birth click Add followed by the number of years/Months/(weeks)/days then click calculate to get the date of death.
- If you want to calculate and age use the Duration Between Two Dates Calculator
The Following is an example of a Date Calculator that is on the Date and Time web page, click on the picture or on the link that is just below the picture to go to a live calculator, have fun doing your own calculating of Dates.
|Depending on the scenario given this calculator might give you two dates, scroll down far enough to see if there are two dates with one being an alternate, if there is, either could be correct, depending on how age was calculated at death. (if unsure which would be the correct date you could enter on documents as day/day, Month, year) For more information and tips: click here .|
online Date Calculator website If a pink box shows up saying error just x it)
Other online calculators
|Age Calculator||Rootsweb||Has both ways to calculate the date of birth and two ways to calculate age||Has both the calendar month method and the 30 day month method (**8870) to chose from.|
|Ancestor Search Age Calculator||searchforancestors.com||Used to determine age from birth date and death date||Uses calendar month method|
|Birth Date Calculator||longislandgenealogy.com||Used to determine birthdate from age at death||Use the calendar month method|
|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. This calendar also allows for an month method or 30 day month method||Has options to use calendar month or 30 day method|
- 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.) 
- 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.
- 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.
How to calculate by hand
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
-Subtract constant 8870
________ 18170927 Born 27 Sept. 1817 (yyyymmdd) 1817yr-09mo-27dd
This person was born on September 27, 1817