Failing tests (Elixir 1.13)

### Steps to reproduce

`mix test`

### Description of issue

> elixir --version
> Erlang/OTP 24 [erts-12.3.2.1] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]

Elixir 1.13.4 (compiled with Erlang/OTP 24)

Constantly failing tests on branch `main` (https://github.com/bitwalker/timex/commit/e1c4d424a5c7840f63efd33cd70e3b5d4f9bfaa1):
```
....................................................................................................................................................

1) test now now/0 returns the duration since epoch (DurationTests)
test/duration_test.exs:137
Expected the difference between -82 and 500000 (500082) to be less than or equal to 500001
code: assert_in_delta(difference_microseconds, 500_000, 500_001)
stacktrace:
test/duration_test.exs:141: (test)

................................................................................................................................................................................................

2) test format RFC3339 (DateFormatTest.FormatDefault)
test/format_default_test.exs:338
Assertion with == failed
code: assert format(date, "{RFC3339}") == {:ok, "0010-09-26T07:00:02+00:50"}
left: {:ok, "0010-09-26T07:00:02+00:53"}
right: {:ok, "0010-09-26T07:00:02+00:50"}
stacktrace:
test/format_default_test.exs:360: (test)
```