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, though it is most likely somewhere in the 200 years of Ood enslavement ending in 4126. It was said in Doctor Who: The Encyclopedia that the story was set in the 42nd century, a a fact confirmed by the Doctor saying in Planet of the Ood that Ood were servants to humans in the 42nd century, thus making it likely that 43K2.1 is in the 42nd century.

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.