As noted by the Ninth Doctor, 12 months made up the Earth year, (TV: World War Three) which was composed of 365 days, 24 hours long. (TV: Last of the Time Lords) The Fourth Doctor remarked that a month lasted 2,678,400 seconds. (TV: The Brain of Morbius)
Months had individual names. Known months — not all of which were Earth months, and so not all of which could possibly equal 2,678,400 seconds — included:
In what the Tenth Doctor termed "a little leeway", the Saxon Master could only have taken the Doctor's TARDIS to as far back as eighteen months prior to its last departure in May 2008. Indeed, the regenerated Master made his first appearance on Earth, under the alias of "Harold Saxon", just after the downfall of Harriet Jones on Christmas 2006. (TV: The Sound of Drums)
The Fourth Doctor believed that Christmas lights "always end[ed] up in an impossible tangle that takes a month and most of your sanity to unpick", and he used this as an analogy to explain to Leela how the unrivalled cosmic storm known as the Great Desolatrix ripped through time and space. (PROSE: Red Planet)
Behind the scenes
- Though he does not specify in the script of The Brain of Morbius, the Doctor's sums reflect a 31-day month.