On Skaro, the radiation level near a Dalek city is measured at 96 units. On the planet Skaro, the 1st Doctor, Susan, Barbara and Ian encounter the Daleks, and help a group of Thals invade their City and immobilize them. I am using the date from James Armstrong's chronology. A Zygon spaceship crashes in Loch Ness, Scotland. Terror of the Zygons.
Exact date is conjecture.
Loch Ness dates back to 565, so they came to Earth before that. This is the first known sighting on record. On the planet Marinus, the Voord and their leader Yartek overcome the Conscience of Marinus and begin to commit numerous crimes. AD 731 Monarch returns to his home world of Urbanka and leaves for Earth for what will be the last time. Four to Doomsday.