Bill Clinton

Bill Clinton was President of the United States in 1997. The Doctor explained the Guardian of the Solar System's role in the Galactic Federation in 3999 to Samantha Jones by comparing it to Clinton's role in the United Nations during Sam's time. The Doctor referred to Clinton as "Billy." (EDA: Placebo Effect)

Behind the scenes
The reference in Placebo Effect to real-world President Clinton introduces a possible discontinuity, as previous and subsequent novels established the fictional Tom Dering as US President during Sam's time.