The API is used to do an update or correction that you would also be able to do in the Enter and Maintain person form.the value you use for p_effective_date is the 'session date' or 'datetrack date' that you use in that form.
So please try and do the same update on this person in the form after datetracking to '01-Feb-2011'.  Is it possible to do this?
It sounds like you person may not exist on that date and so you wouldn't be able to query him up on the screen.  if you cannot do that then the API won't be able to update him on that date either.