Segment of Time

Segment of Time was a time measurement used by people who left Earth on the Ark. The 57th Segment of Time was at least ten million years after the 1st Segment of Time. (TV: The Ark)

The reign of Nero, the Trojan War and wars involving the Daleks were said to be part of the First Segment of Time. (TV: The Ark) The First Doctor was said to look like an inhabitant of the First Segment of Time. (PROSE: Tarnished Image) Shortly after leaving Gallifrey, had crash landed on Destination in the earliest Segments of Time. (AUDIO: The Destination Wars) The Time Lords of the Last Great Time War placed their time scale of Dalek activity, from Davros' childhood in ancient history up to the Battle of the Game Station in the year 200,100, as falling within the First Segment of Time. (PROSE: Dalek Combat Training Manual)

All knowledge of the common cold virus was lost during the Primal Wars of the Tenth Segment of Time. Unsuccessful time travel experiments took place in the 27th Segment of Time. During the 57th Segment of Time, the last humans left on Earth and Monoids evacuated the planet on the Ark to colonise Refusis II. (TV: The Ark)

During the 58th Segment of Time, the planet Metralubit was discovered by human settlers. (PROSE: The Well-Mannered War)

stole from a Farquazi time cruiser during the 300th Segment of Time. (PROSE: The Quantum Archangel)

Behind the scenes
The Discontinuity Guide proposed two alternate definitions of the Segments of Time, the first being that they correspond to a variable length of time. Knowing that the Daleks belong to the first Segment of Time, a suggestion is made that this simply refers to mankind's first overt encounter with them, such that the second segment could begin at some point after the 22nd century Dalek invasion of Earth.

The Primal Wars of the 10th Segment were speculated to refer to the war between the Galactic Federation and Galaxy Five and the time travel experiments of the 27th Segment referring to Magnus Greel's tests in the 51st century. However, if the entirety of Dalek history was encapsulated within the First Segments, then the Second Segment clearly would not begin until after the Dalek Wars, potentially after the 41st century.

Alternatively, if the segments are of roughly equal length, and that the Doctor was correct in his assumption placing the 57th Segment around the year 10,000,000, then the Primal Wars would have taken place nearly two million years into the future, and the time travel experimentation three million years beyond that.