Invalid Moment.js locale from get_user_locale()

[slightlyfaulty]

get_user_locale() returns a locale in the format en_US, which is passed directly to Moment.js, e.g. moment.updateLocale( 'en_US', {...} ).

Since Moment.js doesn't actually support two-part locales, this causes some issues. For example, moment().format( 'Do' ) returns 25 where it should return 25th. This is because the local *ordinal* is lost as a result of setting an undefined locale.

The locale should either be passed in a format that Moment.js understands, such as en, or moment.defineLocale( 'en_US', { parentLocale: 'en' } ) should be called before moment.updateLocale( 'en_US', {...} ). AFAIK these are equivalent.

You can see the issue reproduced here: ​https://codepen.io/slightlyfaulty/pen/WNwpMmG

Of course, swapping out old dusty Moment.js for its super modern and popular drop-in replacement ​Day.js is also an option 😎

[slightlyfaulty]

It might be worth mentioning that this doesn't just affect usage of Moment.js itself, but also the @wordpress/date package.

For example, format( 'jS' ) returns 25 instead of the expected 25th.

[SergeyBiryukov]

Hi there, welcome to WordPress Trac! Thanks for the report.

Some history of the code in question:

• Introduced in [44262] / #45145, changing the version to 5.0 as the earliest applicable version.
• Adjusted in [48075] / #50408 to use moment.updateLocale() instead of moment.locale().

[iandunn]

Related:

• ​https://github.com/WordPress/gutenberg/issues/15221 - Ordinal suffix token does not work in wp.date.format
• ​https://github.com/WordPress/gutenberg/pull/25782 - Use date-fns and date-fns-tz instead of moment