Rework genitive month names in date output

[Rarst]

WP historically does some post-processing on date output for locales that have genitive month names. This is implemented in wp_maybe_decline_date().

The standing implementation is quite brittle since it tried to guess necessary modifications from the output string. So, for example, digits representing year might be confused with digits representing day of months. There is no context to disambiguate.

Likely this needs to be reworked altogether.

The possible approaches are tentatively:

1. Use the date format being processed to determine necessity of replacement.
2. Work the logic into parser code that walks through the format and replaces formats with localized values.

[SergeyBiryukov]

As previously noted in comment:14:ticket:48606, the current implementation fails on formats like F y, where Май 19 ("May 19" in Russian) is assumed to be the 19th day of May and declined, while it's actually the 19th year (as in 2019).

Since wp_maybe_decline_date() is no longer just a filter on date_i18n and is now called directly from wp_date() as a result of [45901], we can pass the date format to it to determine the necessity of replacement, instead of trying to guess the format without any context.

48934.diff​ implements that and adds a new test case.

[Rarst]

A new parameter can't be non-optional for backwards compatibility. We are not in control of how third party code calls the function and that would crash it.

[SergeyBiryukov]

Replying to Rarst:

A new parameter can't be non-optional for backwards compatibility. We are not in control of how third party code calls the function and that would crash it.

Good catch, 48934.2.diff​ checks if the format is passed and falls back to the current implementation otherwise.

[SergeyBiryukov]

In 47076:

Tests: Move wp_maybe_decline_date() tests to the date directory.

See #48934.

[SergeyBiryukov]

In 47078:

Date/Time: Pass the date format to wp_maybe_decline_date().

This ensures that the function has enough context to determine the necessity of replacing the month name with the correct form in locales that require it.

Props SergeyBiryukov, Rarst.
Fixes #48934.

[SergeyBiryukov]

In 47079:

Tests: Update the wp_maybe_decline_date() tests for [47078].

See #48934.

[SergeyBiryukov]

In 47098:

Date/Time: In wp_maybe_decline_date(), add support for a range of days, e.g. February 21–23.

A potential use case is displaying multi-day events in the WordPress Events and News dashboard widget.

See #47798, #48934.