`next()` can return a datetime that is in the past

[MetRonnie]

>>> now = 2020-10-19T13:53Z
>>> next(--10-19)
2020-10-19T00:00Z

is a bug Something is wrong :( because it returns a datetime that is in the past!

It should be

2021-10-19T00:00Z  # next year

Similarly (but less severe), previous() goes back too far

>>> previous(--10-19)
2019-10-19T00:00Z

when it should be

2020-10-19T00:00Z

because just as T00 is short for T0000 and 01 is short for 01T0000, --10-19 is short for --10-19T0000.

I.E. matching date-time components should not be a special case.

Note this is somewhat of a breaking change, so tagged against next minor version rather than maintenance version.

Originally posted by @MetRonnie in #5777 (comment)

[MetRonnie]

We may fix this in metomi/isodatetime#234

[oliver-sanders]

To close this issue, these tests should be fixed:

cylc-flow/tests/unit/cycling/test_iso8601.py

Line 688 in 238ce28

param('next(--08-08)', '20110808T0000Z', marks=pytest.mark.xfail),

cylc-flow/tests/unit/cycling/test_iso8601.py

Line 712 in 238ce28

param('previous(--08-08)', '20100808T0000Z', marks=pytest.mark.xfail),

[hjoliver]

@MetRonnie - bumping this back to 8.4 for now as it seems to have stagnated for a bit.

[MetRonnie]

👍 The fix is ready but has not been a priority for review