How does one go about storing datetime values from a date_select field in the database?

Here is my Ecto model:

defmodule MyApp.User do
  use MyApp.Web, :model

  schema "users" do
    field :birthday, :utc_datetime
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:birthday])
  end
end

Here is my template:

<%= date_select f, :birthday, builder: fn b -> %>
  <%= b.(:day, prompt: "", class: "custom-select") %> /
  <%= b.(:month, prompt: "", class: "custom-select") %> /
  <%= b.(:year, options: Timex.now.year..(Timex.now.year - 100), prompt: "", class: "custom-select") %>
<% end %>

Here is my controller:

def update(conn, %{"user" => user_params}) do
  user = Guardian.Plug.current_resource(conn)
  changeset = User.changeset(user, user_params)
  case Repo.update(changeset) do
    {:ok, _user} ->
      conn
      |> put_flash(:success, gettext("Profile updated!"))
      |> redirect(to: public_profile_path(conn, :show, user))
    {:error, changeset} ->
      conn
      |> render("edit.html", changeset: changeset)
  end
end

I can successfully submit the form, but the birthday is never written to the database