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Fix: preserve datetime precision after Timex.shift/2 #741

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Fix: preserve datetime precision after Timex.shift/2 #741

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1 change: 1 addition & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -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

---

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17 changes: 14 additions & 3 deletions lib/datetime/datetime.ex
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Expand Up @@ -447,13 +447,17 @@ defimpl Timex.Protocol, for: DateTime do
err

%DateTime{} = datetime when shift != 0 ->
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Hope you don't mind if I make further suggestions. How about rewriting the two arrow clauses, like so:

%DateTime{} = original ->
  if shift == 0 do
    original
  else
    original
    |> DateTime.add(shift, :microsecond, Timex.Timezone.Database)
    |> retain_precision(original)
  end

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)
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Maybe it's a good idea to create a function for adding microseconds while keeping the precision? Something like:

original |> add_with_precision_preserved(shift)

|> retain_precision(datetime)

{{ty, _a, _b} = amb, _} when ty in [:gap, :ambiguous] ->
amb
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I would get rid of the case statement below as it seems completely unnecessary, but that may be outside of the scope of this PR. 🤷 😅

Expand All @@ -480,6 +484,13 @@ defimpl Timex.Protocol, for: DateTime do
err
end

defp retain_precision(
%DateTime{microsecond: {ms, _precision}} = 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
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40 changes: 40 additions & 0 deletions test/shift_test.exs
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Expand Up @@ -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