Fix negative microsecond result from DateTime.from_unix/2 (#6599)

Author: jameslong

lib/elixir/lib/calendar/iso.ex

@@ -314,10 +314,11 @@ defmodule Calendar.ISO do
   def from_unix(integer, unit) when is_integer(integer) do
     total = System.convert_time_unit(integer, unit, :microsecond)
     if total in @unix_range_microseconds do
-      microsecond = rem(total, 1_000_000)
+      microseconds = Integer.mod(total, @microseconds_per_second)
+      seconds = @unix_epoch + Integer.floor_div(total, @microseconds_per_second)
       precision = precision_for_unit(unit)
-      {date, time} = :calendar.gregorian_seconds_to_datetime(@unix_epoch + div(total, 1_000_000))
-      {:ok, date, time, {microsecond, precision}}
+      {date, time} = :calendar.gregorian_seconds_to_datetime(seconds)
+      {:ok, date, time, {microseconds, precision}}
     else
       {:error, :invalid_unix_time}
     end

lib/elixir/test/elixir/calendar_test.exs

@@ -237,6 +237,13 @@ defmodule DateTimeTest do
 
     assert DateTime.from_unix(253402300799) == {:ok, max_datetime}
     assert DateTime.from_unix(253402300800) == {:error, :invalid_unix_time}
+
+    minus_datetime = %DateTime{
+      calendar: Calendar.ISO, day: 31, hour: 23, microsecond: {999999, 6},
+      minute: 59, month: 12, second: 59, std_offset: 0, time_zone: "Etc/UTC",
+      utc_offset: 0, year: 1969, zone_abbr: "UTC"
+    }
+    assert DateTime.from_unix(-1, :microsecond) == {:ok, minus_datetime}
   end
 
   test "from_unix!/2" do