Definitely use :utc_datetime_*. In hindsight, we wish we made that the default in Ecto but now we can’t due to backwards compatibility (and there are no plans for Ecto 4)

Here are some resources:

• Don't Do This - PostgreSQL wiki
• Time zones in PostgreSQL, Elixir and Phoenix | AmberBit Sp. z o. o.
• How to save datetimes for future events - (when UTC is not the right answer)

Thank you all so much! this sums it up, once and for all !
So basically it’s just great that Ecto is enforcing saving DateTime values in utc with the utc_datetime, which I feel really positive about since it’s a standard development practice. I’ve seen no other library/framework enforcing it.

I made the following global changes to my Phoenix 1.5 app to use utc_datetime instead of naive_datetime by Ecto, both in schemas and migrations for anyone who may find it useful:

In config.exs, I added this:

config :myapp, MyApp.Repo, migration_timestamps: [type: :utc_datetime]

created a new schema.ex with the following code:

defmodule MyApp.Schema do
  defmacro __using__(_) do
    quote do
      use Ecto.Schema
      @timestamps_opts [type: :utc_datetime]
    end
  end
end

And in my models everywhere, I replaced use Ecto.Schema with use MyApp.Schema

And, done!

This only works for automatically generated fields like inserted_at and updated_at and if you manually set other datetime fields to DateTime.utc_now(). But if you have any other Elixir code that is not timezone-aware and it fetches time in the local timezone (or have a legacy DB with non-UTC datetimes in it) then you’re in for trouble.

Better to do all of these together:

• Use utc_datetime type for datetime fields in your Ecto schema modules.
• When setting times manually, always start with DateTime.utc_now() and/or do calculations with Elixir’s stdlib or timex, always making sure you use UTC.
• If you have to work with an inherited legacy database, just make a one-off script to convert any non-UTC datetimes to UTC. It will save you a world of trouble in the future.