A reference to this confusion appeared in the 2008 episode "The Sontaran Stratagem", where the Doctor was unsure if his time on the UNIT staff took place during the 1970s or the 1980s. Similarly, "The Sarah Jane Adventures" story "The Lost Boy" displays a UNIT file on Sarah Jane Smith which says, "The service quickly expanded, making our presence felt in a golden period that spanned the sixties, the seventies, and, some would say, the eighties." Yet another reference occurred in "The Day of the Doctor", where Kate Stewart refers to "one of my father's incident files. Codename: Cromer. Seventies or eighties depending on the dating protocol". The "Cromer" wording is a reference to the 1972–73-story "The Three Doctors" which, as well as being the first time that Doctors from different eras come together to fight a common foe was the occasion where Brigadier Lethbridge-Stewart, on first being transported to an alien world, likened it to the Norfolk coastal town of Cromer. Kate Stewart again refers to the confusion when she describes events from "Terror of the Zygons" as occurring in the "'70s (or) '80s" ("The Zygon Invasion").