Although no date is ever mentioned, Cpt. Christopher's future son, Col. Sean Jeffrey Christopher, is supposed to lead the first manned mission to Saturn (manned trip to Saturn? WTF?). I'd expect him to be born within a couple of years of "Tomorrow is Yesterday," which would be the early 1970s, which would put him in his mid-30s now. Depending on how old he is when he makes Colonel (or Lt. Col), that should be happening any year now.