There is, and long has been, a disagreement over when the bulk of the UNIT stories were set, specifically whether they were in the 70s or the 80s. This isn't the place to go into the details, but it's been around long enough and has involved enough people to have its own name: The "UNIT dating controversy". Follow the link for more information.
This controversy is so well known that it has even been referenced in passing in the modern version of the show, in "The Sontaran Stratagem" (Donna: What, you used to work for [UNIT]? The Doctor: Yeah, long time ago. Back in the 70’s. Or was it the 80’s?) and in "The Day of the Doctor" (Kate Stewart: "Malcolm, I need you to send me one of my father's incident files. Codenamed Cromer. 70s or 80s depending on the dating protocol.").
"The Enfolded Time" (short story), one of the stories published in the anthology The HAVOC Files as part of the Lethbridge-Stewart series, explained that approximately two decades of time had become "folded" into one, as a side-effect of the Brigadier's frequent contacts with the Doctor. The structure of time was damaged and the confusion over dates experienced by the Brigadier (and others) resulted from this. Thus, the dates really don't make sense & there's an in-universe reason for this.