date_i18n() produces invalid output for shorthand formats

[Rarst]

date_i18n() function relies on parsing passed format to make adjustments. However shorthand formats are not handled and produce invalid output.

Example:

var_dump( date_i18n( 'Y-m-d\TH:i:sP', time() ) ); // 2012-06-15T13:34:03+03:00 << ok
var_dump( date_i18n( DATE_W3C, time() ) ); // 2012-06-15T13:34:03+03:00 << ok
var_dump( date_i18n( 'c', time() ) ); // 2012-06-15T13:34:03+00:00 << broken time zone!

Hook-level fix:

add_filter( 'date_i18n', 'fix_c_time_format', 10, 3 );

function fix_c_time_format( $date, $format, $timestamp ) {

    if ( 'c' == $format )
        $date = date_i18n( DATE_W3C, $timestamp );

    return $date;
}

See ​Why time functions show invalid time zone when using 'c' time format?

Possibly related (can't say for sure from description) #13538

Actually scratch time() from example for more accurate:

var_dump( date_i18n( 'Y-m-d\TH:i:sP' ) );
var_dump( date_i18n( DATE_W3C ) );
var_dump( date_i18n( 'c' ) ); // broken time zone!

Small correction - proper constant for c replacement is DATE_ISO8601, not DATE_W3C (different time zone formatting).

[update] Hm, nope DATE_W3C weirdly seems to be more consistent with c than DATE_ISO8601. While either time zone format is actually ok for standard.