from the DateTime docs:

Developers should avoid creating the DateTime struct directly and instead rely on the functions provided by this module as well as the ones in third-party calendar libraries.

Here’s the helper that I came up with. Only covers the use case I need, doctests included:

@doc """
Ensures that the given `DateTime` has the specified precision.

    iex> ensure_precision(~U[2022-01-01 10:00:00Z], :microsecond)
    ~U[2022-01-01 10:00:00.000000Z]

    iex> datetime = ~U[2022-01-01 10:00:00.123Z]
    iex> DateTime.compare(datetime, ensure_precision(datetime, :microsecond))
    :eq
"""
def ensure_precision(%DateTime{microsecond: {_, 6}} = datetime, :microsecond) do
  datetime
end

def ensure_precision(%DateTime{microsecond: {value, _precision}} = datetime, :microsecond) do
  %{datetime | microsecond: {value, 6}}
end

DateTime.add(datetime, value, :microsecond)

Edit: ignore me, this does not change the precision of the DateTime but perhaps it should.

Doesn’t seem to work for me on Elixir 1.10:

iex> DateTime.utc_now() |> DateTime.truncate(:second) |> DateTime.convert!(Calendar.ISO) |> Map.get(:microsecond)
{0, 0}

I was wrong there. I wasn’t able to find anything.
Sounds like a set of extend/2 functions which take the same arguments as truncate/2 could be helpful.