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) despite that date seemingly being long gone. (TV: Aliens of London et al.) Soon after, by the time the school year began it had been replaced by a red disc that was dated September (TV: Revenge of the Slitheen, The Last Sontaran, The Day of the Clown, The Temptation of Sarah Jane Smith) that at one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) despite the time seemingly being months afterwards. (TV: Aliens of London, Out of Time, The New World)

Others
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)