Fix NaiveDateTime.diff/3 over-counting incomplete units

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 """

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