One of the biggest continuity headaches in Doctor Who is that of UNIT dating. In some ways, this is all William Russell's fault. Why? Let me explain...
We have to go back to the Troughton story The Abominable Snowmen in1967. This story is set in 1935. It proved so successful that a sequel was hurriedly put into production - 1968's The Web of Fear.
Professor Edward Travers (Jack Watling) returned from the earlier serial, now an old man.
He refers to the earlier events taking place "over 40 years" before. That puts The Web of Fear somewhere around the mid to late1970's. This story introduced Brigadier (then Colonel) Lethbridge-Stewart. He returned later that year in The Invasion, UNIT's first outing. The Brigadier says that the Yeti adventure was some 4 years ago - meaning that in UNIT chronology we are at the end of the 1970's.
Producer Derrick Sherwin and his successor Barry Letts have both stated that they believed the UNIT stories to be set in a "near future".
So far, so good.
A problem soon emerges during the Third Doctor exile era. All of the car registration plates are 1970 - 1973, and some calendars appear on screen that imply similar dating. (Maybe all those alien invasions stopped people buying new motors, but doesn't explain not keeping your calendar up to date).
Take The Green Death (1973) for example. This has a very confused chronology. Early on in the story we see a calendar which shows a 29 day month - a leap year February. 1972 when it was filmed, or 1980 when UNIT chronology should place it? (In a later episode we see another calendar showing April. The story takes place over a couple of days only - so what on earth has happened to March this year?).
A 1980 date for this story is reinforced by the arrival of Sarah Jane Smith in the next adventure (The Time Warrior).
In 1975's Pyramids of Mars the Doctor takes her, and Lawrence Scarman, forward in time to see what would happen to the Earth if Sutekh were freed from his captivity - to 1980 to be precise.
Says Sarah: "But I'm from 1980..." Apart from the car number plates and eccentric calendars, everything points so far to near future dating for the UNIT stories. And then Mawdryn Undead happened...
This 1983 story screwed everything up - and it needn't have had William Russell been available. He, of course, played one of the first human companions of the Doctor - school teacher Ian Chesterton - from 1963's An Unearthly Child to 1965's The Chase.
1983 was the 20th anniversary year and producer John Nathan-Turner wanted some element from the past in each story of the season. For Mawdryn Undead it would be an old companion from the first years of the series, in a story set at a Boys' Public School - Ian Chesterton. William Russell proved unavailable, so the Brigadier was drafted in as a replacement. Problem was, they decided not to make any amendments to the story.
The story is set over two time periods - 1977, during the Queen's Silver Jubilee celebrations, and 1983. The 1977 Brigadier says that he retired from UNIT in 1976!!! The Continuity Continuum collapses.
The only way that previous chronology now fits is if both Prof. Travers and Sarah were generalising about dates and were not being specific at all. It means those number plates and calendars were right.
Things suddenly go back to the future with the Brigadier's final story Battlefield in 1989. There is a king, and £5 coins - implying near future once again.
RTD playfully acknowledged the UNIT dating conundrum in their first proper outing of the new series - 2008's The Sontaran Stratagem two-parter.
When Donna learns that the Doctor used to work for UNIT he admits that he did: "Back in the 70's. Or was it the 80's?..." And he quickly moves on.
Present tense or future imperfect? You choose. Me? We can't just pretend that Mawdryn Undead didn't happen, so it has to be contemporary with broadcast I'm afraid.