UNIT dating controversy

The UNIT dating controversy is a problem of retroactive continuity (retcon) which has attracted considerable interest from fans and professional Doctor Who writers alike. It has also been the subject of a 2|entertain DVD documentary, The UNIT Dating Conundrum, included in the special edition release of Day of the Daleks. It has even made its way back into DWU narratives. These have most prominently included a sly remark made by the Tenth Doctor in The Sontaran Stratagem and on-screen graphics seen in The Lost Boy — the latter of which was actually in of itself a reference to the short story UNIT History: Fighting the unknown — but writers in other media have occasionally referenced the controversy, or attempted to solve parts of it.

The essential problem

Though replete with additional nuance, the nub of the narrative problem stems from one serial. Mawdryn Undead tells viewers that Brigadier Alistair Gordon Lethbridge-Stewart retired from UNIT in 1976. However, the events of The Invasion, the first story in which UNIT properly appears, approximately occurred in 1979.^[1] Therefore, the UNIT dating controversy is, broadly speaking, an attempt to understand how the Brigadier could have retired from UNIT before UNIT even existed.

It is worth noting that the original intention of Mawdryn Undead was for Ian Chesterton to return as a teacher. But as William Russell was committed elsewhere and unavailable, it was ultimately decided that the Brigadier would return.

Writer Ben Aaronovitch has notably opined that there is simply no way to retcon the problem.

There is nothing you can do about [Mawdryn Undead]. It's just stuffed. You just pretend it's taking place in an alternate universe . . .Ben Aaronovitch ^{[The UNIT Dating Conundrum [src]]}

Terrance Dicks has said he deliberately avoided giving dates during his time as script editor precisely so he could avoid these sorts of continuity headaches.^[2] Consequently, the biggest period of UNIT involvement, the Third Doctor's era, has only comparatively mild contributions to the dating controversy.

Digging deeper

The problem exists primarily because of two appearances of Alistair Gordon Lethbridge-Stewart, one when he was a colonel in the regular army, and another when he was a public school teacher.

The first was his debut story, The Web of Fear, when he was still a colonel in the regular British Army. Here, he meets the Second Doctor, Jamie, Victoria and crucially, Travers. Victoria says that they met Travers in 1935 in the Himalayas, which they did in The Abominable Snowmen. It is further revealed that 1935 was "over forty years ago", tacitly setting The Web of Fear approximately in 1975 if not after. In a later adventure battling the Cybermen, called The Invasion, the Second Doctor again encounters the newly-promoted Brigadier Lethbridge-Stewart. The Brigadier says their last meeting was "four years ago now", ostensibly putting The Invasion approximately in 1979.

Viewers got no dialogue with a firm year for the whole of the Third Doctor's era. In Pyramids of Mars, however, it is claimed several times that Sarah Jane Smith is from "1980", and she and the Fourth Doctor briefly revisit the year — albeit in an alternative future where the world has been devastated by Sutekh. Since Sarah was the Third Doctor's final companion, and spent a good deal of time at UNIT, this has some implication for the theoretical date of UNIT stories set in seasons 11 - 13.

Whatever the case, her statement is not a clear violation of the continuity established by The Web of Fear. What breaks UNIT dating is Mawdryn Undead. This story firmly and explicitly has the Brigadier retiring from UNIT in 1976, the year The Seeds of Doom was broadcast. The Fifth Doctor confirms that "a year later" from the retirement is 1977, which offers viewers no wiggle room whatsoever. Because it's flatly impossible to have the Brigadier retiring before he's even become the Brigadier, a fundamental discrepancy indisputably exists in televised Doctor Who narratives.

Contradictory clues

There are many other details that confuse the picture.

In The Invasion, Vaughn acquires a photo of the Second Doctor and Jamie from security cameras, which has what appears to be a timestamp at the bottom that reads "E091/5D/78".
Some stories feature calendars, but these can contradict one another. 1973's The Green Death features two such references: one calendar — that in the colliery office — is difficult to read clearly but appears to say it is Thursday the 5th of April. However, it is clearly mentioned in the dialogue that the mine has been out of action for a year. The other — at Global Chemicals — shows Monday the 28th of April, taking the two calendars together, the only possible year the story can be set between 1970 and 1990 is 1980. In 1975's The Android Invasion, the calendar in the pub gives the date every day as "FRIDAY 6 JULY". In 1975, 6 July was a Sunday; the nearest years in which that date fell on a Friday were 1973, 1979, 1984 and 1990.
The Prime Minister in The Green Death, who is only ever seen obliquely, is named as "Jeremy", meaning Jeremy Thorpe of the Liberal Party, which jokingly implies he was going to win the 1974 general election. (The Liberals were a minority party). This would set the story in late 1974 at the earliest, a year after the story aired.
Where politics are concerned, the stories offer a picture very different from when they were transmitted. Jeremy Thorpe was Prime Minister in The Green Death, which he never was in real life. There is a female Prime Minister in 1975's Terror of the Zygons, four years before Margaret Thatcher attained the position; the BBC's website claims this Prime Minister is Shirley Williams, who was never even a party leader in real life. The United Nations is more interventionist than its 1970s real-life counterpart, whilst the Cold War at times is on the verge of turning into World War III in 1971's The Mind of Evil and 1972's Day of the Daleks. However, by 1974's Invasion of the Dinosaurs and 1974/1975's Robot the Cold War is over.
Mao Tse-Tung is implied to be alive at the time of The Mind of Evil, or is at least referred to by Fu Peng as "our Chairman". In real life, he died in 1976, which would date the story before that.
The stories don't usually try to predict future fashions or technology, except when it is central to the plot. The result is that the stories look very strongly like the 1970s. An exception is 1968's The Invasion, where the fashions deliberately don't jibe with then-current fashions, suggesting a near-future setting for that particular story.
In the 1970 serial The Ambassadors of Death, Sergeant Benton comments that the distress signal SOS was done away with "years ago." In the real world, the SOS code was replaced as the standard distress signal in 1999.
The road fund licence (tax) disc on the Doctor's roadster, Bessie, in Robot, is dated to expire in April 1975. All registration year letters on the number plates of fairly new cars in the programmes made in the early-to-mid 1970s are contemporaneous.
Two years before Sarah Jane Smith was travelling with the Fourth Doctor in the 1975 serial The Android Invasion, she had researched a story as a reporter about Britain's "Space Defence Station", which had also apparently been involved in the "first test of the XK-5 space freighter" that made it as far as Jupiter before vanishing. This implies a fairly futuristic space program.
On the occasions that money is mentioned, most amounts given correspond to those in use at the time. 1970's Doctor Who and the Silurians features pre-decimal currency, when a taxi driver asks the Right Honourable Edward Masters for "Seven and six", i.e. seven shillings and sixpence (37½p in decimal currency); while Sarah Jane Smith needs only two pence for a call from a public telephone box in 1976's The Seeds of Doom. In real life, the United Kingdom adopted decimal currency in 1971 and was subject to significant inflation. In Battlefield, which is set around 1997, Pat Rowlinson asks the Doctor and Ace for "five pounds" as the combined costs for a glass of water and a glass of lemonade.
The technology displayed on occasion is more advanced than reality. The United Kingdom has a fully functional space programme that sends missions to Mars and Jupiter (TV: The Ambassadors of Death, The Android Invasion); "disintegrator guns" are in development in 1974/1975's Robot and then "laser guns" are used by UNIT in 1976's The Seeds of Doom; many of the science establishments seen are engaged in extremely advanced research; and there are two artificial intelligences, the first created in the late 1960s. (TV: The War Machines, The Green Death) Most of this technology is not seen in episodes set in the 1980s and onwards. 1968's The Invasion has videophones being used at International Electromatics and by the Ministry of Defence's Major-General Rutlidge. In The Claws of Axos, aired three years later, there is a videophone conversation between Mr Chinn (presumably a high level Ministry of Defence civil servant) and the British Defence Minister. The characters appear to be using standard technology.
Britain has a seventh manned Mars mission and an astronaut corps by the time of The Ambassadors of Death, without a mention of one in previous Earth-set stories. Who Killed Kennedy attributes to cyber-technology recovered after The Invasion but this still means Britain ran seven missions between The Invasion and The Ambassadors of Death; General Carrington was a Mars astronaut in the past and it's implied some time has passed. The Android Invasion says that two years ago, there was a British mission to Jupiter. Both Sarah Jane Smith and Jo Grant both believe that interstellar travel was close to being developed at separate times (Invasion of the Dinosaurs and Colony in Space respectively). The later Remembrance of the Daleks establishes the British Rocket Group operating in 1963; The Dying Days has the Mars Probe series go up to thirteen probes by the late 70s and a fourteenth Mars Probe sent in 1997. However, by the time of The Christmas Invasion (aired 2005 and set in 2006), the British government is boasting about sending an unmanned satellite; in Warriors of Kudlak (aired 2007 and set an undisclosed amount of time after The Christmas Invasion), Sarah Jane says the teenager Lance Metcalf might want to become an astronaut and the first human man on Mars; in Death of the Doctor (aired 2010 and set around that time), UNIT have a moonbase and their own rockets; by The Waters of Mars (aired 2009, set in 2059), humanity still hadn't perfected interstellar travel and wouldn't for about thirty years, a Mars base established the previous year is a highly significant achievement, and Britain lacks a space programme at all. The Tenth Doctor goes as far as to say Bowie Base One — the first off-world base of its kind — is made up of the "very first humans on Mars", though this is complicated by a claim in an on-screen obituary in the episode which states that a three person team, including the base's commander Adelaide Brooke, landed on Mars when Adelaide was 42 years old, in 2041.
The BBC has a third channel, BBC3, in 1971's The Dæmons. In 1971 the BBC had only two channels (though they aspired to launch a third channel in subsequent years). (The actual BBC Three, a digital television channel, was launched in 2003.)
The Seventh Doctor asks Ace (a native of 1987) about the "Yeti in the Underground" and the "Zygon Gambit", meaning if Terror of the Zygons takes place in the 1980s, it's before 1988.
Battlefield (made in 1989) is set in an unspecified near future. The Brigadier has retired from teaching for a few years; he is not many years older than he was in Mawdryn Undead. He's now married, when he lived alone in Mawdryn Undead. Major Husak is from Czechoslovakia, rather than the Czech Republic or Slovakia, but there are also five pound coins and the Brigadier refers to "the King".
In The Sound of Drums, President Arthur Winters tells the Saxon Master that first contact policy that places UNIT in control was decided by the Security Council in 1968, implying UNIT was already active in that year.
In The Sontaran Stratagem, the Doctor says he worked for UNIT in "the 1970s, or was it the '80s?" — a reference to this controversy.
The Fifth Doctor likewise referenced the controversy in the audio story The Gathering, when he requests that Alan Fitzgerald not ask him when he was employed by UNIT.
The Brigadier appears in Enemy of the Bane, an episode dated as occurring around the year 2009 (based upon its setting within the recent Whoniverse); however, the Brigadier is considerably aged for someone who, based on other evidence, would have been active in UNIT only twenty years earlier. In particular, the Brigadier is considerably older than he was in Battlefield, yet stories contemporaneous to Enemy of the Bane establish that the Queen is still on the throne.
In The Day of the Doctor, Kate Stewart asks for one of her father's files; she says it may be filed under the 70s or the 80s "depending on the dating protocol", a clear reference to the UNIT dating controversy. No more is said on screen to explain this so-called protocol or how it works.
In Clara Oswald and the School of Death, the Twelfth Doctor dates the resurfacing of the Sea Devils in The Sea Devils to "1972... or 1982, it gets foggy... your time."
In The Sinestran Kill, set in 1978, Scott Neilson knows of the Doctor as an employee of UNIT from his five years in Special Branch.
In the 1975 TV story Terror of the Zygons, the Brigadier mentions the Navy are "sending some frigates from Chatham" to an underwater object (the Skarasen). In the real world, Chatham Dockyard closed in 1982.
In The Zygon Invasion, Kate Stewart makes a reference to "an attempted Zygon invasion before, in the '70s, '80s," likely referring to the 1975 TV story Terror of the Zygons, and also meaning that that story and later stories' placements are not definitively in the 1980s.

Off-screen evidence

Published books, contemporary interviews, publicity material and behind the scenes documents all point to uncertainty amongst the production team as well.

A document prepared during the making of The Invasion by director Douglas Camfield states that he assumed the story was set in 1976.
The Radio Times programme listing and a continuity announcement at the start of the original transmission of episode one of The Invasion state that the story takes place in 1975. Announcements and publicity material were normally produced by the series' production office, usually by the Script Editor.
In a pair of 1969 interviews, then producer Derrick Sherwin and newly cast Doctor Jon Pertwee told the press that the series (and therefore the UNIT stories) would be set in a near-future time when things such as space stations would have become reality, with Pertwee confirming this would be in the 1980s.
A recorded but unused line in 1971's The Claws of Axos discusses comets due in the period 1969-1975, strongly pointing to an early 1970s setting for the story. By this time, Sherwin had moved on as producer.
The 1972 book The Making of Doctor Who, written by then-script editor Terrance Dicks and regular writer Malcolm Hulke, dates the 1970 story Spearhead from Space to 1970. However the second edition of 1976 (rewritten by Dicks alone, after he had stepped down as script editor) does not specify any date.
The 1974 novelisation Doctor Who and the Sea-Devils, which Malcolm Hulke based on his own The Sea Devils, refers to North Sea oil starting to be exploited in 1978 — indicating an early 1980s setting for the story.
The 1981 Writers' Guide for the proposed series of K9 and Company stated that Sarah's travels with the Doctor (i.e. from The Time Warrior to The Hand of Fear) took place between 1973 and 1976.
According to on-screen production notes for the DVD release of The Time Warrior, a line was struck from the script during Linx's interrogation of Sarah in which she would have explicitly stated her year of origin as 1974.
The UNIT Dating Conundrum looked at the in-universe evidence for the dating of the UNIT stories. It drew the conclusion that there was no reasonable way to reconcile the statements from Mawdryn Undead to those in Third and Fourth Doctor UNIT stories.

Other media

Stories in other media have also offered dates for the UNIT stories but have had little success in producing a clear answer:

Novelisations

The novelisation Doctor Who and the Carnival of Monsters by Terrance Dicks, published in 1977, has the Third Doctor and Jo Grant visiting what they initially believe to be a vessel in the year 1926; Jo states that this was fifty years before her time. The first chapter establishes the story taking place immediately after the events of The Three Doctors, therefore putting that adventure in 1976 (unless Jo was rounding her numbers, which still places it in the 1970s).
The 1993 radio play The Paradise of Death by early 1970s producer Barry Letts is set at the time of the later Third Doctor stories and appears to have a 1990s setting, most notably references to virtual reality. (In the 1994 novelised version, however, Letts limits these references.)
The sequel, 1996's The Ghosts of N-Space, which is set again around the last Third Doctor stories, sees the sighting of a comet which appears every "157 years" and which was last seen in "1818", making the year 1975.
From Downtime, the back cover dates the London Event (The Web of Fear) to 1966 while the narrative dates it to 1968; in either case, it's confirmed that the Event occurred after the Shoreditch Incident — "The Winter of Sixty-three".

Virgin Books

Novels in the Virgin New Adventures and the Virgin Missing Adventures line written in the 1990s took the editorial view that the television stories were set some time in or around the 1970s and left it to individual authors to decide on dates. This resulted in a number of contradictions. Events in The Invasion have been variously dated to the late 1960s, mid 1970s, and late 1970s. (The second chapter of Iceberg is titled "Summer in the 70s".) Battlefield was assumed to take place in 1997.
Transit states the Doctor's exile on Earth was five straight years in the 1970s, so this would span from either Action in Exile or Spearhead from Space to The Three Doctors.
Blood Heat places Spearhead from Space and Doctor Who and the Silurians in 1973.
No Future, set in 1976, mentions the Autons, Axons, and Omega, and dates Zygons to 1975. It also sees the Seventh Doctor erase the Brigadier's memory.
So, things would progress as he knew them. The Brigadier would retire before his time, take up a teaching post without ever really knowing why. That was good, in the end. The old soldier stood for things that wouldn't matter a damn in the next decade. Page 206 ^{[No Future (novel) [src]]}
Who Killed Kennedy, published in 1996, ties several televised stories to real-life 1960s and 1970s political events, like the electoral defeat of the Wilson government. In doing so, it has stories taking place a few months before they aired.
Millennial Rites states that "the aborted attempt to tap geothermal energy" (Inferno) and "the British Space Program" took place in the nineteen-eighties.
In The Scales of Injustice, the Pale Man's statement of how the UK could've helped America in Vietnam by sending over a War Machine seems to be in past tense, implying the story takes place after the Vietnam War, which ended in 1975.
The Dying Days has Mars Probe 13, the last British Mars mission, taking place in 1977. The story is set in 1997.
The Decalog story Prisoners of the Sun is set 20 years or more in the future of an alternate timeline, where Mike Yates has a Radio Times cover dated 23 October 1993, roughly dating Liz Shaw's time in UNIT to c. 1973.
In the Decalog 2: Lost Property story Where the Heart Is, the Third Doctor has been working for UNIT for three years (since Spearhead from Space), and UNIT obtains the manor house that will be their headquarters in The Three Doctors. The Doctor cites The Mutants as his latest work for the Time Lords.

BBC Books

The 1998 BBC Past Doctor Adventures novel The Face of the Enemy, by David A. McIntee, suggests that Mawdryn Undead may take place in a parallel universe where the Brigadier retired in 1976. The novel also reiterates that four years passed between The Web of Fear and The Invasion, and states that Spearhead from Space occurred two years after The Invasion.
The PDA novel The Quantum Archangel repeatedly dates The Time Monster to 1973.
It is stated in the 1999 Eighth Doctor Adventure Revolution Man by Paul Leonard, when Sam Jones was released from prison in 1967, there was a document containing the Brigadier's initials and the UNIT call sign, although both the Eighth Doctor and Sam knew that the Brigadier was still a Colonel and UNIT didn't yet exist.
The Hollow Men seems to date The Dæmons to 1971.
The Christmas Inversion features the Third Doctor, accompanied by Jo Grant and Mike Yates, stumbling upon events contemporary to The Christmas Invasion, which is set on 25 December of 2006. At this point in the "future", the Doctor shows his UNIT pass to a beefeater, only to be told that it had expired thirty years prior.
Deep Blue is set 10-12 years before Tegan's time (i.e. 1983/84) and occurs six months after the events of The Green Death.
The short story Spoilsport in Short Trips: Destination Prague has the President of Czechoslovakia in Jo Grant's time as Svoboda, who was President from 1968 to 1975.

Comics

Change of Mind is dated to 27 January, 1971, with the Third Doctor and Liz Shaw together after she has left UNIT. Hamlet Macbeth is also present, having been discharged from UNIT in December 1968 according to the novel The Left-Handed Hummingbird.
The IDW Publishing comic The Forgotten, first published in 2008, shows the Third Doctor during his exile on Earth with Jo Grant in 1972.
The IDW comic, In With the Tide, first published in 2013, places its Third Doctor and Sarah Jane story in 1974. It also establishes the Cold War, or at least Great Britain being under constant threat of mutually assured destruction, as ongoing, and has the Whomobile, which first appeared in the 1974 TV story Invasion of the Dinosaurs, taken out of mothballs so it can quickly reach UNIT's seabase.
In the comic story Doorway to Hell, printed in Doctor Who Magazine in 2017 and set in 1973, the Delgado incarnation of the Master mentions he returned to Earth following the events of Frontier in Space at the time of the 2016 DWM comic story The Pestilent Heart (set in 1972).

Big Finish

The synopsis for the Big Finish Productions audio play The Coup, released in 2004, places the story in "the near future". Now-General Sir Alistair Lethbridge-Stewart states that UNIT has been fighting alien invasions for forty years, and that he "put down" a Silurian base thirty years before (he is likely rounding up). The story The Longest Night says ICIS's Captain Andrea Winnington was born in the 1980s; this would place it in the late 2000s at the earliest.
- However, PROSE: Rise of the Dominator places the events of The Coup in "the dying days of 2004".
Jo and Iris Wildthyme confirm the former's time at UNIT as being in the 1970s in Find and Replace; this is further confirmed by the Reborn Master in Eyes of the Master that his encounters with the Third Doctor were in the 70s. Later in Defectors, Jo remarks that 1951 is "nearly 30 years ago", setting the story in the late 70s or 1980.
The Fourth Doctor audio dramas The Valley of Death and The Oseidon Adventure have the Brigadier in charge of UNIT (but absent) in 1979. Oseidon itself depicts the second Kraal invasion of Earth, implying The Android Invasion occurred prior.
Trail of the White Worm, also set in 1979 and immediately preceding Oseidon, has the Doctor give Leela a message for UNIT that included references to "Zygons, Krynoids, Axons", indicating that those aliens were encountered by UNIT in or before 1979.
Last of the Cybermen explicitly dates The Invasion to 1975. This would place The Web of Fear in 1971, contradicting Travers's claim that it had been over forty years since 1935. Whatever the date, The Isos Network takes place immediately after Invasion.
As far as determining Liz Shaw's time in UNIT, Shadow of the Past occurs a few weeks after Silurians, The Blame Game "months" after Spearhead, and A Home From Home depicting Liz as being in UNIT for "six months".
As far as determining Jo's time in U.N.I.T, The Other Woman states that she joined in 1970 at the age of 19.
The Power of River Song has Jacqui McGee contacting UNIT in 1971.

Others

History of Mankind according to Doctor Who, a feature in The Terrestrial Index, opted to mostly ignore the "near future" setting of UNIT stories, instead placing them a year following their broadcast. Thus The Web of Fear was placed in 1969, The Invasion in 1970, season 7 in 1971, season 8 in 1972, season 9 in 1973, season 10 in 1974, season 11 and Robot in 1975, Terror of the Zygons in 1976 and The Android Invasion in 1977, allowing the Brigadier to be retired from UNIT that same year in Mawdryn Undead.
The Universal Databank ^[+]Loading...["The Universal Databank (reference book)"] disregarded the near future setting of UNIT stories, specifically the 1980 setting of Pyramids of Mars ^[+]Loading...["Pyramids of Mars (TV story)"]. It placed The Web of Fear in 1969, The Invasion in 1970, with the following stretch of "UNIT era" stories up to The Seeds of Doom ^[+]Loading...["The Seeds of Doom (TV story)"] falling within the 1970s, allowing for the 1977 setting of Mawdryn Undead. The Hand of Fear ^[+]Loading...["The Hand of Fear (TV story)"], in which Sarah Jane Smith leaves the Fourth Doctor, is also stated to take place in 1977.
The Discontinuity Guide disregarded the near future setting of UNIT stories, specifically that of Pyramids. It dated The Web of Fear to 1966, The Invasion to spring of 1969, and the stretch of stories from Spearhead from Space to Terror of the Zygons from 1969 to 1973, allowing the Brigadier to have retired in 1976. Evidence of a contemporary setting is cited with the usage of pre-decimal currency in Doctor Who and the Silurians, Mao Tse-Tung being alive in The Mind of Evil and the 1972 calendar in The Green Death. Technological advances greater than the real world are suggested to have been made with the assistance of Cyber-technology from The Invasion.^[3]
The official U.N.I.T. tie-in website, produced by the BBC webteam to tie into the 2005 revival of Doctor Who, contains various works of fiction that give various dates.
- The short story UNIT History: Fighting the unknown, an in-universe history of UNIT, explicitly mentions that the series of incidents in the London Underground (The Web of Fear) led to the formation of UNIT in 1968. The story also has a joking nod to the controversy over dating of the original stories, as it is stated: "[UNIT] quickly expanded, making our presence felt in a golden period that spanned the sixties, the seventies and, some would say, the eighties." The second, third, and fourth paragraphs of this story, which included the nod to dating controversy, was copied and printed on the prop for Sarah Jane's UNIT dossier in The Lost Boy, an episode of The Sarah Jane Adventures.
The feature UNIT News, published on the U.N.I.T. tie-in website, places the events of Spearhead from Space exactly thirty-five years, to the day, before 25 January 2005 (the concluding episode of Spearhead from Space was broadcast on 24 January 1970).
- It is clear that the fiction on the website dates the original UNIT stories to be contemporaneous with their original broadcast (however, the website dated the post-2005 Doctor Who episodes featuring UNIT to dates unused by any other sources, causing obvious discrepancies).
The 2013 Puffin eshort The Spear of Destiny states directly multiple times that the Third Doctor and Jo work for UNIT in the year 1973, and is set after The Three Doctors.
The 2015 novel The Forgotten Son is set in the immediate aftermath of The Web of Fear and is effectively dated to spring 1969.
In the 2017 anthology Tales of Terror, the short story The Monster in the Woods occurs on Halloween 1973, where the the Third Doctor is still in exile and Day of the Daleks has already happened. In the next story, Toil and Trouble, the Fourth Doctor dates Terror of the Zygons to "1975. Or thereabouts."
The Whoniverse states that Fury from the Deep, presumably dated to 1968, occurred before the formation of UNIT.
The 2020 novel Time Traveller's Diary says that UNIT was founded on 29 May "in the 1970s…or possibly the 1980s".

Solving the controversy

While many stories have tried to give specific dates to a serial, very few have offered an overall answer to the problem.

In Marc Platt's novelisation of Downtime, Christopher Rice states that there were no "official" records of the Brigadier while discussing UNIT:

'Colonel Alastair Gordon Lethbridge-Stewart figures largely in its setting up in the Seventies. He's a Brigadier by then with a remarkable active service record.' He leant forward slightly. 'And then he vanishes.'
'You must have known him during your time at UNIT,' said Miss Waterfield.
Sarah shook her head wearily. 'Sorry to disappoint you, but I'm just a journalist.'
'With no "official" records of him, you could easily imagine Lethbridge-Stewart was dead,' continued Christopher.
The Vice Chancellor half turned to him. 'UNIT looks after its own.' Page 100 ^{[Downtime (novelisation) [src]]}

The notion of UNIT personnel being protected later comes up in the audio Find and Replace, where it's stated there was a fear of enemies coming after personnel after they'd left UNIT. In that story the Third Doctor attempted to alter Jo Grant's memories as a way of protecting her.

The Virgin New Adventure The Dimension Riders introduces the concept of crystallized time, where alterations to the timeline usually manifested as small things like dates changing their days, which could account for such discrepancies between the DWU and the real world.

In the Eighth Doctor Adventure novels Interference - Book One and Interference - Book Two, published in 1999, Sarah Jane Smith is uncertain whether her experiences with the Doctor and UNIT take place in the seventies or the eighties. At the end of the "What Happened on Earth" portion of the second novel, she asks the Doctor about this, and he replies "Temporal slippage. ... My fault, I'm afraid. I think it's currently the 1970s, but —" and is interrupted by Sam Jones.

This solution was later reinforced by The Enfolded Time, where the Accord explains that the Doctor's regular visits to Earth from 1969 to 1989 in his faulty TARDIS disrupted the planet's timeline, enfolding time in on itself so that twenty years occurred over a ten-year period and the Accord is trying to repair the damage to strengthen it before a forthcoming war. The obvious mix around of some of this history is then handled by the dating protocols.

In The Split Infinitive, the '60s and '70s are said to be being dragged together when a group of Rocket Men are trapped in the respective eras, crunching the years down. Ace explains that this will have caused UNIT's timeline to be scrambled due to their association with the Doctor and other time-travellers, and the Seventh Doctor explicitly references the Brigadier retiring in the '70s and working in the '80s as a symptom of this damage.

Sources

Parkin, Lance, Doctor Who: A History of the Universe - From Before The Dawn of Time and Beyond The End of Eternity (London, UK: Virgin Publishing, 1996), ISBN 0-426-20471-9
Miles, Lawrence & Wood, Tat, About Time: The Unauthorized Guide to Doctor Who - 1970-1974, Seasons 7 to 11 (New Orleans, LA: Mad Norwegian Press, 2004), ISBN 0-9725959-2-9

Footnotes

↑ The Radio Times programme listing for The Invasion episode one sets the story even earlier, as the synopsis begins: "The Tardis brings Dr. Who, Jamie and Zoe back to England, but England about 1975." (original published text)
↑ [[The UNIT Dating Conundrum (documentary)|]]
↑ https://www.bbc.co.uk/doctorwho/classic/episodeguide/datingunit.shtml

[1] The Radio Times programme listing for The Invasion episode one sets the story even earlier, as the synopsis begins: "The Tardis brings Dr. Who, Jamie and Zoe back to England, but England about 1975." (original published text)

[2] [[The UNIT Dating Conundrum (documentary)|]]

[3] ttps://www.bbc.co.uk/doctorwho/classic/episodeguide/datingunit.shtml

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