31st century

According to Eldane, the 31st century was the time his branch of homo reptilia would awaken from the slumber into which they placed themselves in 2020. It therefore was a time of renewed contact between his species and humans. (TV: Cold Blood)

According to the Fifth Doctor, the 31st century was "all space cities and monorails." (AUDIO: The Butcher of Brisbane)

The 31st century was the start of a period of long term societal collapse for both Earth and Kustollia following "an enormous, terrible battle" between the humans of Earth and the Kustollons of Kustollia, as recalled by Igrix and the Ninth Doctor. (COMIC: The Love Invasion)

Behind the scenes
According to Lance Parkin's Whoniverse: An Unofficial Planet-By-Planet Guide to the Universe of the Doctor, from Gallifrey to Skaro, the script for Into the Dalek specifies that the story takes place in the 31st century.