I saved this formula for calculating birth dates from another list and I claim neither ownership nor accuracy: REMEMBER THE NUMBER 8870 This is not an error. It is the number to remember when you want to find the birthdate of someone when you only have the date of death and age. Suppose the person died May 6, 1889, at the age of 71 years, 7 months, 9 days. 1- Write the month day year as 18890506 2- Subtract the age at death 710709 3- That gives you the figure 18179797 4- Now subtract 8870 8870 5- The result is 18170927 year 1817, 9th month (Sept), 27th day, or 27 September, 1817 Source Given: Platte Co. MO Historical/Genealogical Society