Microsecond-precision test failures

### Steps to reproduce

Running `mix test` with Elixir 1.18 after updating dependencies so that they will compile (see #772).

### Description of issue

- What are the expected results? All tests pass in current versions of Elixir.

I'm not entirely sure what the desired behaviour is for these tests, so I'm hoping someone more familiar with this code base can take a look or provide some direction.

```
1) test shift to zero (ShiftTests)
test/shift_test.exs:175
Assertion with === failed
code: assert expected === result
left: ~N[0000-01-01 00:00:00]
right: ~N[0000-01-01 00:00:00.000000]
stacktrace:
test/shift_test.exs:178: (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)

3) test add (TimexTests)
test/timex_test.exs:14
Assertion with === failed
code: assert expected === result
left: ~U[2015-07-02 14:27:52Z]
right: ~U[2015-07-02 14:27:52.000000Z]
stacktrace:
test/timex_test.exs:18: (test)

4) test subtract milliseconds (TimexTests)
test/timex_test.exs:35
Assertion with === failed
code: assert subtracted.microsecond === {900_000, 2}
left: {900000, 6}
right: {900000, 2}
stacktrace:
test/timex_test.exs:39: (test)

5) test subtract (TimexTests)
test/timex_test.exs:28
Assertion with === failed
code: assert expected === result
left: ~U[2015-06-16 14:27:52Z]
right: ~U[2015-06-16 14:27:52.000000Z]
stacktrace:
test/timex_test.exs:32: (test)
```