This conversion happens within ActiveRecord when you save your model.
You could work around it with something like this:
somedate = DateTime.new(params["date1(1i)"].to_i,
params["date1(2i)"].to_i,
params["date1(3i)"].to_i,
params["date1(4i)"].to_i,
params["date1(5i)"].to_i)
DateTime::new
is an alias of DateTime::civil
(ruby-doc)