UNIT DATES THAT ARE ESTABLISHED:
In the 1968 story The Web of Fear Professor Travers says that 1935 was "over forty years ago", thus making it at least 1975. In The Invasion, the first story to feature UNIT, it is said that the events of The Web of Fear took place "about four years ago", making it 1979 at the earliest. This would place the UNIT stories featuring the Third Doctor and the Fourth Doctor in the 1980s.
In the 1973 story Carnival of Monsters, the Third Doctor's companion Jo says that 1926 is "about forty years" earlier than her own time. This would place the Third Doctor UNIT stories in the 1960s.
In the 1975 story Pyramids of Mars, the Fourth Doctor's companion Sarah states, "I'm from 1980". This would place the Third and Fourth Doctor UNIT stories in the late 1970s.
In the 1981 spin-off K-9 and Company Sarah has been back on Earth for some years, with the Doctor having left a present for her in 1978. This would place the relevant UNIT stories in the mid 1970s at the very latest.
In the 1982 story Time-Flight, which has a contemporary setting, the Fifth Doctor wonders if Brigadier Lethbridge-Stewart has become a General by now, implying that it is several years since his time in UNIT.
In the 1983 story Mawdryn Undead it is established that Lethbridge-Stewart retired in 1976 (and was not promoted to a General) and worked at a British public school from 1977 until at least 1983. The story features two timezones 1977, which features celebrations of the Silver Jubilee of Queen Elizabeth II, and 1983, which is repeatedly confirmed as taking place "six years" later.
In the 1989 story Battlefield the Brigadier has now retired from teaching and the Seventh Doctor tells his companion Ace (who is from the late 1980s) that they are "a few years in (her) future". The serial includes a few subtle hints (mostly relating to technology such as telephones) that it takes place later than the year of its transmission.
