Forgive me if I do not use quotes.  I am pulling this from memory and would have it listed as general statements instead of direct quotes. In Space Seed, Spock does say that in the late 1990's a group of these young supermen took control.  But in the same seen he also says records of that time period are sketchy at best.  Adding would you reveal to a war weary world that some 80 or 90 of these young supermen were still unaccounted for. With the statement that the records of that time period are sketchy at best, could allow the actual time for Khan to be shifted about 100 years from the 1990 stated in the TOS episode after further research. Within the _frame_work of TOS, certainly. But the sketchy records in the later shows prove to include near-complete family records for the cryo-stored not-quite-celebrities from the 1980s in TNG The Neutral Zone ; the complete works of Dixon Hill; and just about every little detail of the Bell Riots, including (misattributed) photos. So sketchy either has to mean that the material existed in great detail but had simply not been sorted out by the time of Space Seed , or that there were real lacunae in it but those only affected very short periods of time, not an entire century. To be sure, all these detailed historical records we hear about are from the time after Space Seed . Perhaps time-travel missions like the one Kirk pulled in Assignment: Earth were crucial in getting the sketchy records clarified? Perhaps there was nothing sketchy about the past after these missions? A whole century's worth of history or more could have been completely rewritten, and the rewrite accepted as the uncontested truth from there on. Timo Saloniemi