hi, im confused here, i have my grandfathers name but no date, his name was john porter! my dad was born in 1944, sooo , to get my granspa john's birthdate i add or subtract 20 yrs?. & after i do that i would know my grandpa john's birthdate?. then do i add or subtract another 20 yrs to find out my grt grandpa's date of birth then??. little confussed her on this!!.