Elixir: Date and time

defmodule Test do
  use ExUnit.Case

  describe "shift_days work" do
    test "with Date" do
      time = Date.utc_today()

      assert Date.add(time, -2) == Solution.shift_days(time, -2)
      assert Date.add(time, 2) == Solution.shift_days(time, 2)
      assert Date.add(time, 1) == Solution.shift_days(time, 1)
      assert Date.add(time, 0) == Solution.shift_days(time, 0)
    end

    test "with NaiveDateTime" do
      time = NaiveDateTime.utc_now()

      assert NaiveDateTime.add(time, days_to_seconds(-2), :second) ==
               Solution.shift_days(time, -2)

      assert NaiveDateTime.add(time, days_to_seconds(2), :second) == Solution.shift_days(time, 2)
      assert NaiveDateTime.add(time, days_to_seconds(1), :second) == Solution.shift_days(time, 1)
      assert NaiveDateTime.add(time, 0, :second) == Solution.shift_days(time, 0)
    end

    test "with DateTime" do
      time = DateTime.utc_now()

      assert DateTime.add(time, days_to_seconds(-2), :second) == Solution.shift_days(time, -2)
      assert DateTime.add(time, days_to_seconds(2), :second) == Solution.shift_days(time, 2)
      assert DateTime.add(time, days_to_seconds(1), :second) == Solution.shift_days(time, 1)
      assert DateTime.add(time, 0, :second) == Solution.shift_days(time, 0)
    end

    test "with Time" do
      time = Time.utc_now()

      assert Time.add(time, days_to_seconds(-2), :second) == Solution.shift_days(time, -2)
      assert Time.add(time, days_to_seconds(2), :second) == Solution.shift_days(time, 2)
      assert Time.add(time, days_to_seconds(1), :second) == Solution.shift_days(time, 1)
      assert Time.add(time, 0, :second) == Solution.shift_days(time, 0)
    end

    defp days_to_seconds(amount) do
      amount * 24 * 60 * 60
    end
  end
end