From aaeb55c1c9645201947cb801c2c85a9f9261b2b7 Mon Sep 17 00:00:00 2001
From: nseaSeb <nseaprotector@gmail.com>
Date: Fri, 10 Jul 2026 16:55:14 +0200
Subject: [PATCH] Fix `NaiveDateTime.diff/3` over-counting incomplete units

`NaiveDateTime.diff/3` truncated each operand to the requested unit and
only then subtracted, i.e. it computed `floor(t1) - floor(t2)` instead of
`floor(t1 - t2)`. When the subtrahend's sub-second fraction exceeded the
minuend's, this rounded the result *up*, contradicting the documented
guarantee that "Fractional results are [...] truncated" and the example
that it "rounds incomplete days to zero".

For instance an elapsed span of 86399.5s (less than one day) returned `1`
for `:day`, and a 0.5s span returned `1` for `:second`. The sibling
`DateTime.diff/3`, which carries the identical doc line, returned `0` for
the same wall-clock instants because it computes the difference at the
`:microsecond` base first and truncates once via `System.convert_time_unit/3`.

Align `NaiveDateTime.diff/3` with `DateTime.diff/3` so both agree on
identical instants. All existing doctests are preserved.

Assisted-by: Claude Code:claude-opus-4-8
---
 lib/elixir/lib/calendar/naive_datetime.ex     |  8 ++++---
 .../elixir/calendar/naive_datetime_test.exs   | 23 +++++++++++++++++++
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/lib/elixir/lib/calendar/naive_datetime.ex b/lib/elixir/lib/calendar/naive_datetime.ex
index df37473ee04..0c7af910141 100644
--- a/lib/elixir/lib/calendar/naive_datetime.ex
+++ b/lib/elixir/lib/calendar/naive_datetime.ex
@@ -570,9 +570,11 @@ defmodule NaiveDateTime do
             "unsupported time unit. Expected :day, :hour, :minute, :second, :millisecond, :microsecond, :nanosecond, or a positive integer, got #{inspect(unit)}"
     end
 
-    units1 = naive_datetime1 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(unit)
-    units2 = naive_datetime2 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(unit)
-    units1 - units2
+    diff_microsecond =
+      (naive_datetime1 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(:microsecond)) -
+        (naive_datetime2 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(:microsecond))
+
+    System.convert_time_unit(diff_microsecond, :microsecond, unit)
   end
 
   @doc """
diff --git a/lib/elixir/test/elixir/calendar/naive_datetime_test.exs b/lib/elixir/test/elixir/calendar/naive_datetime_test.exs
index 14e724dad3c..7c2f02f21aa 100644
--- a/lib/elixir/test/elixir/calendar/naive_datetime_test.exs
+++ b/lib/elixir/test/elixir/calendar/naive_datetime_test.exs
@@ -221,6 +221,29 @@ defmodule NaiveDateTimeTest do
 
       assert NaiveDateTime.diff(%{dt | second: 57}, dt, :second) == 50
     end
+
+    test "truncates the elapsed difference, not each operand (matches DateTime.diff)" do
+      # The true elapsed span is 0.5s (< 1s) and 86399.5s (< 1 day). Sub-second
+      # fractions must not inflate the count: per the "rounds incomplete days to
+      # zero" guarantee, and consistent with the sibling DateTime.diff/3.
+      assert NaiveDateTime.diff(
+               ~N[2000-01-01 00:00:01.200000],
+               ~N[2000-01-01 00:00:00.700000],
+               :second
+             ) == 0
+
+      assert NaiveDateTime.diff(
+               ~N[2000-01-02 00:00:00.200000],
+               ~N[2000-01-01 00:00:00.700000],
+               :day
+             ) == 0
+
+      assert NaiveDateTime.diff(
+               ~N[2000-01-01 00:00:00.001200],
+               ~N[2000-01-01 00:00:00.000700],
+               :millisecond
+             ) == 0
+    end
   end
 
   test "convert/2" do