4221

43K2.1 was the year during which the Doctor encountered the Walker Expedition on the planet Krop Tor. (DW: The Satan Pit)

It is unknown what date 43K2.1 refers to, although it may refer to the year 43,231 CE, or possibly the year 4321 CE. We don't know when the Walker Expedition had left Earth or how long it had stayed on Krop Tor by the time its crew members met the Doctor and Rose.

But 43K2.1 is likely to be in the 42nd century, as it was confirmed in Doctor Who: The Encyclopedia and The Satan Pit was set before Planet of the Ood.