Tax disc

A tax disc was a form of vehicle licensing used in the United Kingdom. The date on them indicated their expiry date and were replaced when that happened.

Sarah Jane Smith
The tax disc on Sarah Jane's car changed often. When fighting the Bane she possessed a purple disc that was due to expire in July 2007. (TV: Invasion of the Bane) Soon after, by the time the school year began it had been replaced by a red disc that was dated September (TV: Revenge of the Slitheen, The Last Sontaran, The Day of the Clown, The Temptation of Sarah Jane Smith) which at one point faced backwards. (TV: Eye of the Gorgon)

By the time of her second encounter with Mrs Wormwood and the Bane, she had replaced the red disc with another of the same colour that had hints of gold numbering which was dated July. (TV: Enemy of the Bane) By 2009, the disc was replaced yet again with one light blue in colour that read September 2009. (TV: Prisoner of the Judoon, The Wedding of Sarah Jane Smith, The Gift)

In 2010, the disc was replaced with a purple one. (TV: The Nightmare Man, Death of the Doctor, The Vault of Secrets) The following year, it was replaced a final time with another red one that expired in March 2011. (TV: Sky)

Torchwood and their operatives
The Torchwood SUV did not possess a tax disc. (TV: Everything Changes et al.)

The car that Gwen Cooper drove with Suzie Costello at the time of her resurrection had a red tax disc that expired in March 2007. (TV: They Keep Killing Suzie) A different car she drove soon after whilst investigating the death of Eugene Jones possessed a blue disc dated April 2007. (TV: Random Shoes)

Owen Harper's car had a light blue tax disc. (TV: Out of Time)

Others
On 26 June, Ian Chesterton convinced himself that he was back in his own time after seeing White City tube station and a tax disc dated December 1965. (TV: "The Planet of Decision")

The car that the Men in Black used did not possess a tax disc. (TV: The Vault of Secrets)

Peter Warmsly's tax disc in 1997 read 30.6.99. (PROSE: Battlefield)

In early March 2005, Mickey Smith's tax disc was due to expire on the 31 March that year. (TV: Rose)

The Preachers' van had a tax disc that was due to expire on 10 April 2006. (TV: The Age of Steel)

A military jeep that was involved in a skirmish against the Cybermen had a blue tax disc due to expire in July. (TV: Doomsday)

The Pete Tyler of Pete's World had an old car with a tax disc that was due to expire in June. (TV: Doomsday)

Behind the scenes
The use of tax discs in stories filmed months before airing contribute to the Aliens of London dating controversy, in which various Russell T Davies executive produced stories set in modern times are meant to be set a year after the first airing of the episode based on the plot of Aliens of London, in which Rose Tyler has been away from Earth (from her family's perspective) for a year. This is particularly notable with the discs changing between Invasion of the Bane and Revenge of the Slitheen, as Luke Smith is meant to be recently activated, and "born yesterday, near enough" at the start of the latter story, not at least two months old.