Calculating Dates for Genealogy

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

Calculating Dates Using Online Calculators
There are two ways to calculate a date from an age at death:
 * 30 day month method; example: 28 Apr 1864
 * exact month method; example: 30 Apr 1864

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 month method: 30 Apr 1864 30 day month method: 28 Apr 1864

Which Way is Better?
That just depends on how you wish to calculate the date. The 30 day month method assumes that all months have 30 days, whereas the exact month method uses the exact amount of dyas for each month.

Although, the exact month method would be more accurate when calculating a date. Some believe that the dates and ages calculated and found on tombstones uses the 30 day month method.

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.

Using the Date Calculator Website

 * 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. (there are other types of Calculators on this page)



online Date Calculator website If a pink box shows up just x it)

To discover additional Date calculator links and tips: click here

How to Calculate a Date Manually
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

So how does one calculate a date to get as close as possible to the date of birth? By using a Date Calculator that works the way genealogical records work - which is almost always by first subtracting the years, then subtracting the months, then subtracting the weeks, then subtracting the days.

NOTE: some date calculators use the reverse order, which will calculate a different date in many scenarios!

To check whether a calculator works for genealogy an example would be: Death: 13 Mar 1870, Age: 5 years 10 months, 13 days, Birthday: 30 Apr 1864. (An incorrect date calculator will show 28 Apr 1864 - do not use it or you will be off by days on your calculations.)


 * Each calculator uses a different interface, so the instructions are calculator-specific. In general, you want to insert the death date and then ask the calculator to Subtract the number of Years, Months, (weeks if given), Days.


 * If 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, you can use this tested-working Date Calculator

To discover additional Date calculator links and tips: click here