`plstlib`: Raise `ValueError` instead of `TypeError` when loading a partial ISO 8601 date

Bug report

Bug description:

Bug report

plistlib is documented (in the code) to accept ISO 8601 <date> values where the smaller units are omitted: the date regex _dateParser makes the month, day and time components optional, and the comment above it says "Smaller units may be omitted with a loss of precision".

However, when a <date> value actually omits the day or month, loading fails with a confusing internal TypeError instead of producing a datetime:

>>> import plistlib
>>> plistlib.loads(b'<plist><date>2024-06Z</date></plist>')
Traceback (most recent call last):
  ...
TypeError: datetime() missing required argument 'day' (pos 3)
>>> plistlib.loads(b'<plist><date>2024Z</date></plist>')
Traceback (most recent call last):
  ...
TypeError: datetime() missing required argument 'month' (pos 2)

Dates that omit only the time components work fine:

>>> plistlib.loads(b'<plist><date>2024-06-07Z</date></plist>')
datetime.datetime(2024, 6, 7, 0, 0)
>>> plistlib.loads(b'<plist><date>2024-06-07T08Z</date></plist>')
datetime.datetime(2024, 6, 7, 8, 0)

The cause is in Lib/plistlib.py, _date_from_string(): it builds the positional argument list for datetime.datetime() by stopping at the first missing component, so for 2024-06Z it only passes (2024, 6) — fewer than the three required arguments (year, month, day).

The omitted components should default to the start of the period (month=1, day=1, hour/minute/second=0), consistent with how omitted time components already behave and with the documented "loss of precision".

I have a fix with a regression test ready (#151217).

CPython versions tested on:

3.16, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-151217
• gh-151285
• gh-152958
• gh-153244

Start in Lib/plistlib.py at _date_from_string() and review the _dateParser comment and matching behavior for partial ISO 8601 dates. Use the examples in the issue as regression cases, then check linked PR #151217 and the other linked work before proceeding; done means omitted month, day, and time components load without the reported TypeError.