When UNIT (The United Nations Intelligence Taskforce) was created in (the real world in) 1968, it was the deliberate decision of the producers/writers to set the UNIT stories approximately a decade in the future. Lethbridge-Stewart first appears in "The Web of Fear" which is explicitly set over forty years after 1935. "The Invasion" is four years after that, and then "Spearhead from Space" is several months after "The Invasion". In "The Invasion" a photograph is dated to December 1978. In interviews Producer Derek Sherwin stated that the UNIT stories were set in "the future", while star Jon Pertwee stated that the stories were set in the 1980's. The novelisation of the Season 9 story "The Sea Devils" referred to 1977 as having been years ago. Things like manned Mars missions were believed to going to take place c. 1980. The politics, technology etc. All clearly represented what people in 1969/1970 believed the 1980's were going to be like.
However in 1983, new Producer John Nathan Turner featured the same Lethbridge-Stewart character who stated that he had retired from UNIT in 1976, and had been teaching at a boys' school since 1977, six years before. Obviously this is totally incompatible with everything that had been established about UNIT and UNIT dating. Many people took the viewpoint that as the predicted future had never materialised, that version of the 1980's was null and void. They pointed to the 1970's clothes and hairstyles, as well as 1970's social attitudes, prices being consistent with 1970's prices, the actual date never explicitly being stated onscreen during the Pertwee UNIT years.
This has created a major rift in fandom. Do the UNIT stories take place in the 1980's as was clearly intended by the people who actually created UNIT, or do they take place in the 1970's as explicitly stated onscreen in "Mawdryn Undead"? Various spin-off novels, audios, comics, short stories have come down firmly on one set of dating, others have deliberately avoided dating UNIT stories at all, and television stories have had the Doctor be confused whether he was with UNIT in the 70's or 80's, or suggested UNIT dating protocol may have more than one way of dating events.
As has been stated, taking the position that the UNIT stories are set in one decade will be backed up by one set of evidence, while being totally contradicted by another set of evidence. Saying matter-of-factly that they took place in (choose one) the 1970's/1980's shows that you are ignoring some evidence and making a statement on canon. Various unofficial guidebooks (such as The Discontinuity Guide, About Time or Timelink) have claimed to have provided conclusive evidence to one set of dating. However, they have chosen to simply ignore one set of evidence, and horrifically twist the meanings of simple English words to get to their conclusions. Everything from characters being dimwitted, to parallel universes, to the effects of the Time War have been suggested.
In the end certain UNIT stories are explicitly set in the 1970's. And certain UNIT stories are explicitly set in the 1980's. There is no rational way to reconcile them. And nobody can realistically claim to have "the answer".
In "The Enfolded Time" (short story), published in the limited-edition (300 copies) anthology The HAVOC Files as part of the Lethbridge-Stewart series, an entity called the Accord explained to Brigadier Lethbridge-Stewart that approximately two decades of time had become "folded" into one, as a side-effect of the Brigadier's frequent contacts with the Doctor. Thus, the structure of time was damaged and the confusion over dates experienced by the Brigadier (and others) resulted from this. In the story, the Accord was repairing the structure of time because "We foresee a great war spreading throughout time which will do uncountable damage." The repairs didn't remove the confusion over dates but did make the Brigadier (and others) aware of the confusion and of the need for "new dating protocols" to take account of it.
This story doesn't attempt to reconcile the discrepancies; instead, it makes them a real feature of the DWU: the dates don't make sense because time got messed up.