In the event you don't have a computer close at hand to do the calculating:

REMEMBER 8870

That's not an error: It's 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.

Write the year, month, day as:

18890506

Subtract the age at death:
710709
This gives the figure:
18179797
Now subtract 8870:
8870
The result is:
18170927
Year 1817, 9th month (Sept), 27th day or 27 Sept, 1817  

From Platte Co, MO Historical/Genealogical Society

Other helpful websites:

http://searchforancestors.com/utility/birthday.html

http://www.dhc.net/~design/search.htm#CALC

http://www.cyndislist.com/calendar.htm

http://www.icubed.com/~ldill/utilities/index.html

Here's a shareware program you can download.
http://lombardo.simplenet.com/tombston.zip