Calculating a Birthdate without a calculator

To do this calculation by hand when the online calculator is not available, use the 8870 formula.(the calculator calls this the 30 day month length)

If a person died May 6, 1889 at age 71 years, 7 months and 9 days:

18890506
Year, day, month of death (yyyymmdd)
-710709 Subtract age at death (yymmdd)
_________
18179797

-8870 Subtract constant 8870
________
18170927 Born 27 Sept. 1817 (yyyymmdd)

This person was born on September 27, 1817

JUST REMEMBER 8870 and it works everytime!