I know Davros created the Daleks and that story was told in Genesis of the Daleks, but when were they created, in relation to Earth's time?
There is no definitive answer to this.
In A History of the Universe, it's dated at 4000 B.C.. This book is followed by AHistory (An Unauthorized History of the Doctor Who Universe), where (in the second edition, the date is revised to 760 A.D. based on the Dalek Chronicles TV Century 21 comic strip (dubious canon, in my opinion). The Doctor Who: The Dalek Handbook has it even later, in 1500 A.D.
Complicating matters further, there are two contradictory Dalek origin stories (although some have tried to unify these through alternate timeline stories). As well as Genesis of the Daleks, there was the very early (second serial of the 1st Doctor) The Daleks, which also had no confirmed date (possibly 1963, the year the Doctor was aiming for; 2263 in AHistory; 2290 according to Doctor Who Monthly #75).