[simpleqt]

format_interval() raised NotImplementedError for every skeleton whose CLDR interval pattern defines the era (G) or the day period (B), because DateTimeFormat.extract() only understood y M d H h m a:

>>> from babel.dates import format_interval, format_skeleton
>>> format_skeleton('Gy', datetime(2020, 1, 1), locale='en')                      # works
'2020 AD'
>>> format_interval(datetime(2020, 1, 1), datetime(2021, 1, 1), 'Gy', locale='en')
NotImplementedError: Not implemented: extracting 'G' field from pattern Gy

interval_formats defines those fields for every locale, so 13 skeletons are affected everywhere — Gy, GyM, GyMd, GyMEd, GyMMM, GyMMMd, GyMMMEd, GyMMMM, GyMMMMd, GyMMMMEd, GyMMMEEEEd, Bh and Bhm — 1,121 locales in all.

This extracts the two fields the same way they are rendered: the era index as format_era() computes it, and the day-period identifier from format_period(). The return annotation widens to int | str, matching the existing a case, and the value picked is the one for the field the two instants differ in, so the output follows the CLDR pattern selected for it:

en  Gy    2016 – 2017 AD
en  GyM   1/2016 – 3/2017 AD
en  Bh    11 in the morning – 4 in the afternoon
en  Bhm   11 in the morning – 4 in the afternoon

A sweep over all locales, all interval skeletons and a set of instants (230,698 cases) goes from 20,220 NotImplementedErrors to none, with 20,132 of those cases returning output instead of raising and no case whose output changed — nothing that used to succeed formats differently now.

tests/test_date_intervals.py::test_format_interval_era_and_day_period covers the four skeletons above: 4 failed before, 14 passed after in that file. The full suite is 1 failed / 7,831 passed with the change (the remaining failure, test_setuptools_commands, is environment-only and fails on the default branch too).

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