"8870"

Gravestones often supply only the Date of Death and Age.
A swift way to calculate the date of birth is as follows:

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

1. Write the Death date year, month, day as............. 18890506

2. Subtract the age at death.......................................... 710709

3. This gives the figure ............................................ 18179797

4. Now subtract 8870 ....................................................8870

5. The result is .......................................................18170927

Which reads as year 1817, 9th month (September), 27th day

September 27, 1817

Do you believe this is always true?