Tombstone Birthday Calculator Tips All dates should be considered approximate due to possible errors in the original calculation found on the tombstone, obituary or death record. Always note in your records that your date is calculated. You can use either the exact month method or the 30 day month method to calculate a birthday. Please note, you might think the exact calendar month method is more reliable, but realize that the age on many of the headstones and in the obituaries seem to have been calculated using the 30 day month system. Remember! A tombstone or death certificate is not considered a primary source for a birthdate, but it can give you an approximate date to help further your research looking for the birth record This calculator could also be used to calculate the birth year from the census records or from the wedding date and age. The calculator can accept Julian calendar dates, but you can’t mix Julian and Gregorian dates or ages in the same equation. Because of the change from the Julian Calendar to the Gregorian Calendar, the calculator may not work properly for anyone born between Sept. 2, 1752 and September 14, 1752 … (this could be from days that are lost at the changeover period of the calendar. Know when the calendar was changed from Julian to Gregorian in the country where your ancestor was born or died. You can check this site for more information: Anchestor Search There is a formula that you can use when an online calculator is not available. It’s called the 8870 method. If a person died May 6, 1889 at age 71 years, 7 months and 9 days: 18890506    Year, day, month of death (yyyymmdd)    -710709    Then you would subtract the age at death in this manner (yymmdd). _________ 18179797         -8870    Subtracting the constant number 8870 ________ 18170927    born on: 1817-09-27 (yyyy-mm-dd) yyyymmdd Calculation denotes that this person was born ca Sept. 27, 1814.