This episode did not air in Los Angeles until December 22, 1973 due to George Takei's run for L.A. City Council. Takei's opponents claimed that if any episode that featured him aired before the election, it would violate the equal time law in which political candidates must have an equal amount of air time on non-news programs. For that region, Yesteryear (1973) was the premier episode of this series.
This episode was first shown in public during the World Science Fiction convention (Torcon 2) in Toronto, 31 Aug - 3 Sept.
This is the second time a non-corporeal alien would "become the Enterprise". In Return to Tomorrow (1968), one named Sargon did it as well.
Kirk refers to the timespan that the reactor is functioning as "all these millenniums." While the most common plural of "millennium" is "millennia," "millenniums" is an accepted plural.