Howling:The Daleks in "Day of the Daleks"

The Daleks in "Day of the Daleks" evidently come from a time many centuries in the future, as they recognise the First and Second Doctors, but not the Third. So if they conquered Earth sometime in the late 20th/early 21st century, they must have travelled back in time from their home era......but surely that would cause a massive paradox in their history, likely eradicating the future they came from? 82.2.136.93 19:25, September 7, 2011 (UTC)