When searching for information on older tombstones frequently the birth dates are not present. Several years ago I took a beginner genealogy class at our local community college. During one of the sessions the instructor gave a presentation on just this topic. He introduced the class to the tombstone birthday calculator. I have been using the calculator from the Ancestor Search site with great success, this is how they describe the calculator:
"Many older tombstones list age and date of death. This tool calculates birth date from the age of death and the date of death on the tombstone, death certificate or obituary. Results are given in a year-month-day format."
Below is an example of a tombstone that does have all the information listed. Notice how the calculator was off by just one day.
To do the calculations by hand use what is called the 8870 formula:
If a person died May 6, 1889 at age 71 years, 7 months, and 9 days:
-710709 Subtract age at death (yymmdd)
-8870 Subtract constant 8870
18170927 Born 27 September 1817 (yyyymmdd)
This person was born on September 27, 1817