Month

A month was a unit of time measurement. According to the Fourth Doctor — who was almost certainly speaking of its duration on Earth — a month lasted 2,678,400 seconds. (TV: The Brain of Morbius) 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)

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: allowcachedresults=true category=DWU months columns=6

In what the Tenth Doctor termed "a little leeway", 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)

Behind the scenes

 * Though he doesn't specify in the script of The Brain of Morbius, the Doctor's sums reflect a 31-day month.