Source

File: wp-includes/functions.php

function wp_maybe_decline_date( $date, $format = '' ) {
	global $wp_locale;

	// i18n functions are not available in SHORTINIT mode.
	if ( ! function_exists( '_x' ) ) {
		return $date;
	}

	/*
	 * translators: If months in your language require a genitive case,
	 * translate this to 'on'. Do not translate into your own language.
	 */
	if ( 'on' === _x( 'off', 'decline months names: on or off' ) ) {

		$months          = $wp_locale->month;
		$months_genitive = $wp_locale->month_genitive;

		/*
		 * Match a format like 'j F Y' or 'j. F' (day of the month, followed by month name)
		 * and decline the month.
		 */
		if ( $format ) {
			$decline = preg_match( '#[dj]\.? F#', $format );
		} else {
			// If the format is not passed, try to guess it from the date string.
			$decline = preg_match( '#\b\d{1,2}\.? [^\d ]+\b#u', $date );
		}

		if ( $decline ) {
			foreach ( $months as $key => $month ) {
				$months[ $key ] = '# ' . preg_quote( $month, '#' ) . '\b#u';
			}

			foreach ( $months_genitive as $key => $month ) {
				$months_genitive[ $key ] = ' ' . $month;
			}

			$date = preg_replace( $months, $months_genitive, $date );
		}

		/*
		 * Match a format like 'F jS' or 'F j' (month name, followed by day with an optional ordinal suffix)
		 * and change it to declined 'j F'.
		 */
		if ( $format ) {
			$decline = preg_match( '#F [dj]#', $format );
		} else {
			// If the format is not passed, try to guess it from the date string.
			$decline = preg_match( '#\b[^\d ]+ \d{1,2}(st|nd|rd|th)?\b#u', trim( $date ) );
		}

		if ( $decline ) {
			foreach ( $months as $key => $month ) {
				$months[ $key ] = '#\b' . preg_quote( $month, '#' ) . ' (\d{1,2})(st|nd|rd|th)?([-–]\d{1,2})?(st|nd|rd|th)?\b#u';
			}

			foreach ( $months_genitive as $key => $month ) {
				$months_genitive[ $key ] = '$1$3 ' . $month;
			}

			$date = preg_replace( $months, $months_genitive, $date );
		}
	}

	// Used for locale-specific rules.
	$locale = get_locale();

	if ( 'ca' === $locale ) {
		// " de abril| de agost| de octubre..." -> " d'abril| d'agost| d'octubre..."
		$date = preg_replace( '# de ([ao])#i', " d'\\1", $date );
	}

	return $date;
}