Darkest Days of the Time War

The Darkest Days of the Time War was the name given to some of the worst days faced by those in the Last Great Time War, (PROSE: Secrets of Time Lord Victorious) a conflict fought between the Daleks and Time Lords that was often compared to living through Hell. (TV: The End of Time, et. al) However, according to a Data Worm that broke into the Archive of Islos, not even those alive during these dark days could manage to break into the Dark Times of the universe. Additionally, despite its name claiming to be the War's "Darkest Days", (PROSE: Secrets of Time Lord Victorious) those alive in the conflict were faced with the fact that recognisable days no longer existed, (AUDIO: Day of the Master) though the term "day" was still often used throughout the fighting. (TV: The End of Time, et. al)

Behind the scenes
The publisher's summary for the novelisation of The Day of the Doctor establishes the Time War's "dark days" to be its final days, describing the War Doctor fighting Daleks during the time.