How to merge conditions hash with parameters

report_controller.rb

  def gps_activity
    @sidebar_units = Unit.where(:fleet_id => @chosen_fleets_array).not_hidden.order("NAME")

    unless params[:unit_ids].blank?
      @chosen_units_array = params[:unit_ids]
    else
      @chosen_units_array = @sidebar_units.collect{ |u| [u.id]}
    end

    conditions = {:fleet_id => @chosen_fleets_array}

    unless params[:datetime_start].blank? and params[:datetime_end].blank? then
      dt_start = DateTime.strptime(params[:datetime_start], "%m/%d/%Y %H:%M").to_i
      dt_end = DateTime.strptime(params[:datetime_end], "%m/%d/%Y %H:%M").to_i

      #conditions.merge!( :gps_timestamp => " >= #{dt_start}" )
      #conditions.merge!( :gps_timestamp => " <= #{dt_end}" )
    end

    #@histories = GpsHistory.where(conditions).order("gps_timestamp DESC")

    @histories = GpsHistory.where(conditions).
        where("gps_timestamp >= ?", dt_start).
        where("gps_timestamp <= ?", dt_end).
        order("gps_timestamp DESC")


    #SELECT "gps_histories".* FROM "gps_histories"
    #WHERE "gps_histories"."fleet_id" IN (4000)
    #AND (gps_timestamp >= 1321959600)
    #AND (gps_timestamp <= 1321963200)
    #ORDER BY gps_timestamp DESC

  end