Timex.shift/2 with time-shift always converts precision to :microsecond in Elixir v1.14+

[enerick]

Steps to reproduce

with Elixir v1.13

iex(1)> now = ~U[2023-01-01 10:20:30Z]
~U[2023-01-01 10:20:30Z]
iex(2)> Timex.shift(now, hours: 1)
~U[2023-01-01 11:20:30Z] # OK

with Elixir v1.14

iex(1)> now = ~U[2023-01-01 10:20:30Z]
~U[2023-01-01 10:20:30Z]
iex(2)> Timex.shift(now, hours: 1)
~U[2023-01-01 11:20:30.000000Z] # precision changes unnecessarily

Description of issue

• Elixir v1.14 changes DateTime.add/4 behaviour to preserve the maximum precision of given parameters.
  • commit: elixir-lang/elixir@5a583c7
  • relates to: Time.add behaviour changed in 1.14 elixir-lang/elixir#12303
• Timex.shift/2 uses DateTime.add/4 with :microsecond precision to apply time-shift result.

  • timex/lib/datetime/datetime.ex

    Line 450 in e1c4d42

    DateTime.add(datetime, shift, :microsecond, Timex.Timezone.Database)

This causes unnecessary precision conversions in Elixir v1.14 and later.

[aniravi24]

That change went live in Elixir 1.14.3, so the versions prior to that work fine still.

[railsmechanic]

Same happend to NaiveDatetime, as it converts the NaiveDateTime to DateTime under the hood.

timex/lib/datetime/naivedatetime.ex

Lines 238 to 240 in e1c4d42

	def shift(%NaiveDateTime{} = datetime, shifts) when is_list(shifts) do
	with {:ok, dt} <- DateTime.from_naive(datetime, "Etc/UTC", Timex.Timezone.Database) do
	case Timex.shift(dt, shifts) do

It uses the Timex.shift/2 (which operates with DateTime), which leads to the following result:

iex(1)> now = ~N[2023-03-31 10:31:57]
iex(2)> Timex.shift(now, hours: 1)
~N[2023-03-31 11:31:57.000000]

This breaks some of my code (using ecto), with the error "...naive_datetime expects microseconds to be empty...", where just ~N[2023-03-31 10:31:57] is expected. Is this the supposed behavior in the future?

The intended behaviour is to only add microseconds when subsecond precisions is requested. https://github.com/elixir-lang/elixir/blob/5a583c753b96865a7cdec2fb4c1ab9c96b836d24/lib/elixir/lib/calendar/naive_datetime.ex#L379-L387