Add unit fast paths to Calendar.ISO.iso_days_to_unit/2 (#15771)

Assisted-by: Claude Code:claude-fable-5

Signed-off-by: Thomas Cioppettini <544875+tomciopp@users.noreply.github.com>

Author: tomciopp

lib/elixir/lib/calendar/iso.ex

@@ -2115,10 +2115,24 @@ defmodule Calendar.ISO do
   end
 
   @doc false
-  def iso_days_to_unit({days, {parts, ppd}}, unit) do
-    day_microseconds = days * @parts_per_day
-    microseconds = divide_by_parts_per_day(parts, ppd)
-    System.convert_time_unit(day_microseconds + microseconds, :microsecond, unit)
+  def iso_days_to_unit(iso_days, :second) do
+    floor_div_positive_divisor(iso_days_to_microseconds(iso_days), @microseconds_per_second)
+  end
+
+  def iso_days_to_unit(iso_days, :millisecond) do
+    floor_div_positive_divisor(iso_days_to_microseconds(iso_days), 1_000)
+  end
+
+  def iso_days_to_unit(iso_days, :microsecond) do
+    iso_days_to_microseconds(iso_days)
+  end
+
+  def iso_days_to_unit(iso_days, unit) do
+    System.convert_time_unit(iso_days_to_microseconds(iso_days), :microsecond, unit)
+  end
+
+  defp iso_days_to_microseconds({days, {parts, ppd}}) do
+    days * @parts_per_day + divide_by_parts_per_day(parts, ppd)
   end
 
   @doc false

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

@@ -491,6 +491,21 @@ defmodule DateTimeTest do
     assert DateTime.to_unix(min_datetime) == -377_705_116_800
   end
 
+  test "to_unix/2 with non-default units floors pre-epoch datetimes" do
+    datetime = ~U[1969-12-31 23:59:59.999999Z]
+
+    assert DateTime.to_unix(datetime, :second) == -1
+    assert DateTime.to_unix(datetime, :millisecond) == -1
+    assert DateTime.to_unix(datetime, :microsecond) == -1
+    assert DateTime.to_unix(datetime, :nanosecond) == -1_000
+
+    assert DateTime.from_unix!(-17_412_508_654_473, :millisecond)
+           |> DateTime.to_unix(:millisecond) == -17_412_508_654_473
+
+    assert DateTime.from_unix!(1_000_000_123, :millisecond)
+           |> DateTime.to_unix(:millisecond) == 1_000_000_123
+  end
+
   test "compare/2" do
     datetime1 = %DateTime{
       year: 2000,