cycling: last day of the month - 31T00

[oliver-sanders]

One might expect the sequence 31T00 to yield...

YYYY-01-31T00:00
YYYY-02-28T00:00
YYYY-03-31T00:00
YYYY-04-30T00:00
...

...whereas in actuality it produces:

YYYY-01-31T00:00
YYYY-02-28T00:00
YYYY-03-28T00:00
YYYY-04-28T00:00
...

Strangely 0231T00 yields nothing and more strangely still 0331T00 results in YYYY-03-02T00:00.

This seems a little inconsistent.

[oliver-sanders]

03-31T00 resulting in 03-02T00 is definitely worthy of the "bug" label, incidentally R1/20000331T00 works fine!

[matthewrmshin]

🐛 it is.

[arjclark]

Note that R1/0331T00 doesn't work, suggesting that the problem may be down to partial date expansion.

[dvalters]

R1/0329T00 works fine. It seems not to like any combination of month with 30th or 31st day. (Except January...)

(Incidentally, is it supposed to be leap year aware? R1/010229T00 expands to a non-existent date 2001-02-29)

[arjclark]

Incidentally, is it supposed to be leap year aware? R1/010229T00 expands to a non-existent date 2001-02-29

I think, in practice, that should either never be run or be interpreted as 2001-03-01 (not clear to me which)

[oliver-sanders]

Bug is in the isodatetime library, see metomi/isodatetime#80.

[MetRonnie]

It is not clear to me how cylc is going from 2001-01-31 to 2001-02-28 since in isodatetime:

2001-01-31 + ---31 = 2001-01-31
2001-02-01 + ---31 = 2001-03-31

[MetRonnie]

It is not caused by metomi/isodatetime#80 as I have come up with a fix for that and the bug persists here.

Btw I think the sequence 31T00 should only yield the months which have 31 days

2000-01-31
2000-03-31
2000-05-31
...

Whereas it is perfectly possible to use 01T00-P1D, however this is still victim to the bug

2000-01-31
2000-02-29
2000-03-29
...
2001-01-29
2001-02-28
2001-03-28
...

[MetRonnie]

The bug is here - the interval is naively taken from the next unit up from the largest truncated property, so 31T00 results in an interval of P1M, and 2000-01-31 + P1M = 2000-02-29

cylc-flow/cylc/flow/time_parser.py

Lines 330 to 345 in 474f21d

	def _get_interval_from_expression(
	self, expr: Optional[str], context: Optional['TimePoint'] = None
	) -> Optional[Duration]:
	if expr is None:
	if context is None or not context.truncated:
	return None
	prop_name = context.get_largest_truncated_property_name()
	kwargs = {}
	if prop_name == "year_of_century":
	kwargs = {"years": 100}
	if prop_name == "year_of_decade":
	kwargs = {"years": 10}
	if prop_name in ["month_of_year", "week_of_year", "day_of_year"]:
	kwargs = {"years": 1}
	if prop_name == "day_of_month":
	kwargs = {"months": 1}

[arjclark]

@MetRonnie - well done tracking that one down