Fix: preserve datetime precision after Timex.shift/2

Resolves #731

Unfortunately this broke with elixir-lang/elixir@5a583c7
which was release with Elixir 1.14.3

Elixir 1.13.4:

```
iex(3)> DateTime.utc_now() |> IO.inspect() |> DateTime.truncate(:second) |> IO.inspect() |> Timex.shift(minutes: 1) |> IO.inspect()
~U[2023-04-13 09:56:10.136274Z]
~U[2023-04-13 09:56:10Z]
~U[2023-04-13 09:57:10Z]
```

Elixir 1.14.4:

```
iex(1)> DateTime.utc_now() |> IO.inspect() |> DateTime.truncate(:second) |> IO.inspect() |> Timex.shift(minutes: 1) |> IO.inspect()
~U[2023-04-13 09:55:16.405357Z]
~U[2023-04-13 09:55:16Z]
~U[2023-04-13 09:56:16.000000Z]
```

CHANGELOG.md

@@ -16,6 +16,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 - Updated `Timex.now/1` typespec to remove the `AmbiguousDateTime`
 - Corrected pluralization rules for bg/cs/he/id/ro/ru
+- Fixed `Timex.shift/2` to preserve the precision of the provided datetime
 
 ---
 

lib/datetime/datetime.ex

@@ -447,13 +447,17 @@ defimpl Timex.Protocol, for: DateTime do
           err
 
         %DateTime{} = datetime when shift != 0 ->
-          DateTime.add(datetime, shift, :microsecond, Timex.Timezone.Database)
+          datetime
+          |> DateTime.add(shift, :microsecond, Timex.Timezone.Database)
+          |> retain_precision(datetime)
 
         %DateTime{} = datetime ->
           datetime
 
-        {{ty, _, _}, %DateTime{} = orig} when ty in [:gap, :ambiguous] and shift != 0 ->
-          DateTime.add(orig, shift, :microsecond, Timex.Timezone.Database)
+        {{ty, _, _}, %DateTime{} = original} when ty in [:gap, :ambiguous] and shift != 0 ->
+          original
+          |> DateTime.add(shift, :microsecond, Timex.Timezone.Database)
+          |> retain_precision(datetime)
 
         {{ty, _a, _b} = amb, _} when ty in [:gap, :ambiguous] ->
           amb
@@ -480,6 +484,13 @@ defimpl Timex.Protocol, for: DateTime do
       err
   end
 
+  defp retain_precision(
+         %DateTime{microsecond: {ms, _prevision}} = new_datetime,
+         %DateTime{microsecond: {_ms, precision}} = _original_datetime
+       ) do
+    %{new_datetime | microsecond: {ms, precision}}
+  end
+
   defp logical_shift(datetime, []), do: datetime
 
   defp logical_shift(datetime, shifts) do

test/shift_test.exs

@@ -229,4 +229,44 @@ defmodule ShiftTests do
     expected = ~D[2017-12-31] |> Timex.to_datetime()
     assert expected === date
   end
+
+  describe "DateTime does not change precision" do
+    test "seconds" do
+      datetime = Timex.shift(~U[2023-04-13 08:00:00Z], minutes: 1)
+      expected = ~U[2023-04-13 08:01:00Z]
+      assert expected === datetime
+    end
+
+    test "milliseconds" do
+      datetime = Timex.shift(~U[2023-04-13 08:00:00.000Z], minutes: 1)
+      expected = ~U[2023-04-13 08:01:00.000Z]
+      assert expected === datetime
+    end
+
+    test "microseconds" do
+      datetime = Timex.shift(~U[2023-04-13 08:00:00.000000Z], minutes: 1)
+      expected = ~U[2023-04-13 08:01:00.000000Z]
+      assert expected === datetime
+    end
+  end
+
+  describe "NaiveDateTime does not change precision" do
+    test "seconds" do
+      datetime = Timex.shift(~N[2023-04-13 08:00:00Z], minutes: 1)
+      expected = ~N[2023-04-13 08:01:00Z]
+      assert expected === datetime
+    end
+
+    test "milliseconds" do
+      datetime = Timex.shift(~N[2023-04-13 08:00:00.000Z], minutes: 1)
+      expected = ~N[2023-04-13 08:01:00.000Z]
+      assert expected === datetime
+    end
+
+    test "microseconds" do
+      datetime = Timex.shift(~N[2023-04-13 08:00:00.000000Z], minutes: 1)
+      expected = ~N[2023-04-13 08:01:00.000000Z]
+      assert expected === datetime
+    end
+  end
 end