Date Calculator
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
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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
Other online calculators
| Links | Website | Description |
|---|---|---|
| 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 |
Tips
- 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
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