Gregorian calendar

The Gregorian calendar was a dating system used on Earth, with 365.25 days to the year. (PROSE: Instead of You) Consequent to the British changeover from the Julian calendar to the Gregorian, eleven days — 2 September through 14 September 1752 — were "lost". These days, as experienced in London, were purchased by the Faction Paradox and became their heartland, known as the Eleven-Day Empire. (PROSE: Interference - Book One)

At the ceremony held in honour of the 50th anniversary of the October Revolution in Red Square in Moscow on 7 November 1967, the Seventh Doctor explained to his companion Ace that the celebration of the October Revolution was held in November as Russia was still using the Julian calendar, which was eleven days behind the Gregorian calendar, in 1917. (AUDIO: Thin Ice)