Fixes #18.
Fixes #171.

The Python docs say:

String representations of timedelta objects are normalized similarly to their internal representation. This leads to somewhat unusual results for negative timedeltas. For example:

>>> timedelta(hours=-5)
datetime.timedelta(days=-1, seconds=68400)
>>> print(_)
-1 day, 19:00:00

However, we assumed all components have the same sign, and used absolute values of some of those, leading to inconsistent behaviour (on Linux and macOS; it still worked on Windows).

Instead, we should convert the whole timedelta into its absolute value, and then use its components directly.

Also test all the timedeltas with positive and negative values.