I found this on http://olivetreegenealogy.com website. How do you figure a birthdate? Suppose the person died May 06, 1889, at the age of 71 years, 7 months, 9 days. Write the year, month, day as: 18890506 (1889, May, 06) Subtract the age at death: 710709 (71 yrs, 7 mos, 09 days) This gives the figure: 18179797 Now subtract 8870: 8870 The result is: 18170927 (1817/09/27 1817 September 27)