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.
How do you figure the birthdate?

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

1. Write the 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

Year 1817, 9th month (Sept), 27th day or 27 Sep 1817