this is what i dont get u say add 20 or 40 yrs to my dads brithdate what would be 1944 +20 yrs would be+ 1964! it dont make since?> not if i take my dads birth yr, 1944 & subtract 20 yrs, i get 1944-20 yrs, =1924!. heres the kicker my mom was born in 1947 - 20 yrs =1907 but my her dad was born in 1906!!that i know im just confussed here!. for sure!!. now if i take my dad's birth yr. & subtract20 yrs. i get 1924 for my grandpa john's birth yr!!. then if i take 1924 subtract 20 yrs again this would give me 1904 for my grt graandfathers birth yr./??/ or do i subtract 40 from my dads birth yr to get my grandfathers birth yr?>!! confussed here!! someone please help me!!!.