Do not truncate microseconds in DateTime.diff/3 (#14274)

Closes #14273

lib/elixir/lib/calendar/datetime.ex

@@ -1599,11 +1599,13 @@ defmodule DateTime do
     end
 
     naive_diff =
-      (datetime1 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(unit)) -
-        (datetime2 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(unit))
+      (datetime1 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(:microsecond)) -
+        (datetime2 |> to_iso_days() |> Calendar.ISO.iso_days_to_unit(:microsecond))
 
     offset_diff = utc_offset2 + std_offset2 - (utc_offset1 + std_offset1)
-    naive_diff + System.convert_time_unit(offset_diff, :second, unit)
+
+    System.convert_time_unit(naive_diff, :microsecond, unit) +
+      System.convert_time_unit(offset_diff, :second, unit)
   end
 
   @doc """

lib/elixir/test/elixir/calendar/datetime_test.exs

@@ -807,6 +807,24 @@ defmodule DateTimeTest do
       # Test with a non-struct map conforming to Calendar.datetime
       assert DateTime.diff(Map.from_struct(dt1), Map.from_struct(dt2)) == 3_281_904_000
     end
+
+    test "diff with microseconds" do
+      datetime = ~U[2023-02-01 10:30:10.123456Z]
+
+      in_almost_7_days =
+        datetime
+        |> DateTime.add(7, :day)
+        |> DateTime.add(-1, :microsecond)
+
+      assert DateTime.diff(in_almost_7_days, datetime, :day) == 6
+    end
+
+    test "diff in microseconds" do
+      datetime1 = ~U[2023-02-01 10:30:10.000000Z]
+      datetime2 = DateTime.add(datetime1, 1234, :microsecond)
+
+      assert DateTime.diff(datetime1, datetime2, :microsecond) == -1234
+    end
   end
 
   describe "from_naive" do