Bette Butcher Topp provided the following simple formula for determining a birthdate when you know the date of death and age. It's called "Formula 8870."

Example:  Died 6 May 1889 		18890506
		  Age 71 years 7 mos. 9 days	  710709
						_________
						18179797
						   -8870
						________
						18170927