I have a application built on Phoenix 1.3. I use timex and timex_ecto in order to deal with datetimes with time zone.

  defp deps do
    [
      {:phoenix, "~> 1.3.3"},
      {:phoenix_pubsub, "~> 1.0"},
      {:phoenix_ecto, "~> 3.2"},
      {:postgrex, ">= 0.0.0"},
      {:phoenix_html, "~> 2.10"},
      {:phoenix_live_reload, "~> 1.0", only: :dev},
      {:gettext, "~> 0.11"},
      {:cowboy, "~> 1.0"},
      {:timex, "~> 3.1"},
      {:timex_ecto, "~> 3.2"}
    ]
  end

The schema module MyApp.Schedule.Item is defined as follows:

defmodule MyApp.Schedule.Item do
  use Ecto.Schema
  use Timex.Ecto.Timestamps

  schema "items" do
    field(:name, :string)
    field(:starts_at, Timex.Ecto.DateTime)
  end
end

In the priv/repo/seeds.exs, I inserts a record like this:

import MyApp.Repo
time0 = Timex.now("Asia/Tokyo") |> Timex.beginning_of_day()

insert!(%MyApp.Schedule.Item{
  name: "Example",
  starts_at: Timex.shift(time0, days: 1, hours: 10)
})

I need some adivice to upgrade my application to Phoenix 1.4 (rc.3) and Ecto 3.0.

Firstly, I removed timex_ecto from mix.exs because timex_ecto does not work with Ecto 3.0.

  defp deps do
    [
      {:phoenix, "~> 1.4.0-rc"},
      {:phoenix_pubsub, "~> 1.1"},
      {:phoenix_ecto, "~> 4.0"},
      {:ecto_sql, "~> 3.0-rc"},
      {:postgrex, ">= 0.0.0-rc"},
      {:phoenix_html, "~> 2.11"},
      {:phoenix_live_reload, "~> 1.2-rc", only: :dev},
      {:gettext, "~> 0.11"},
      {:jason, "~> 1.0"},
      {:plug_cowboy, "~> 2.0"},
      {:timex, "~> 3.1"}
    ]
  end

Secondry, I rewrote the MyApp.Schedule.Item module as follows:

defmodule MyApp.Schedule.Item do
  use Ecto.Schema

  schema "items" do
    field(:name, :string)
    field(:starts_at, :utc_datetime)
  end
end

Then, the priv/repo/seeds.exs came to cause this error:

** (ArgumentError) :utc_datetime expects the time zone to be "Etc/UTC", got `#DateTime<2018-11-03 11:00:00+09:00 JST Asia/Tokyo>`

Of course, to fix this error I can rewrite the script like this:

time0 = 
  Timex.now("Asia/Tokyo") 
  |> Timex.beginning_of_day()
  |> Timex.Timezone.convert("Etc/UTC")

insert!(%MyApp.Schedule.Item{
  name: "Example",
  starts_at: Timex.shift(time0, days: 1, hours: 10)
})

But, I think such a change is a little burdensome, because my app has quite a lot similar expressions.

So, taking advice in Compile error with ecto 3.0 and timex_ecto 3.3 · Issue #2770 · elixir-ecto/ecto · GitHub, I created a custom Ecto type for my application:

defmodule MyApp.Ecto.DatetimeWithTimezone do
  @behaviour Ecto.Type
  def type, do: :datetime

  def cast(%DateTime{} = dt), do: {:ok, dt}
  def cast(_), do: :error

  def load(%NaiveDateTime{} = ndt), do: DateTime.from_naive(ndt, "Etc/UTC")
  def load(_), do: :error

  def dump(%DateTime{} = datetime) do
    case Timex.Timezone.convert(datetime, "Etc/UTC") do
      %DateTime{} = dt -> {:ok, dt}
      {:error, _} -> :error
    end
  end
  def dump(_), do: :error
end

Then, I made following modifications on my Model.User module:

defmodule MyApp.Schedule.Item do
  use Ecto.Schema
  # use Timex.Ecto.Timestamps           # <- Removed
  alias MyApp.Ecto.DatetimeWithTimezone # <- Added

  schema "users" do
    field(:name, :string, null: false)
    # field(:starts_at, Timex.Ecto.Datetime) # <- Removed
    field(:starts_at, DatetimeWithTimezone)  # <- Added
  ...

With these changes, my app works just like before.

Am I upgrading my app in a correct way? Or should I basically change its design?