I made a mistake in my earlier posting on the "Family Tree." I meant to say that the Don and Carlo didn't die in '54, as was listed in the "Tree." 1955 is the date of their deaths--the key year when Michael settled all family business. It's confirmed in the Don's burial scene. Look for a closeup on his tombstone, which reads, "CORLEONE Vito April [xx] 1887, July [xx] 1955." '55 is confirmed during the baptism/massacre scene, when you see Clemenza, clutching his gift-wrapped shotgun, get into a '55 Cad.
The tombstone appears to put the lie to the "Tree"'s listing of 1892 as Vito's birthdate, but the child Vito who flees Sicily in 1901 looks more like a 9-year-old than a 14-year-old.