Source

File: wp-includes/functions.php

function date_i18n( $format, $timestamp_with_offset = false, $gmt = false ) {
	$timestamp = $timestamp_with_offset;

	// If timestamp is omitted it should be current time (summed with offset, unless `$gmt` is true).
	if ( ! is_numeric( $timestamp ) ) {
		$timestamp = current_time( 'timestamp', $gmt );
	}

	/*
	 * This is a legacy implementation quirk that the returned timestamp is also with offset.
	 * Ideally this function should never be used to produce a timestamp.
	 */
	if ( 'U' === $format ) {
		$date = $timestamp;
	} elseif ( $gmt && false === $timestamp_with_offset ) { // Current time in UTC.
		$date = wp_date( $format, null, new DateTimeZone( 'UTC' ) );
	} elseif ( false === $timestamp_with_offset ) { // Current time in site's timezone.
		$date = wp_date( $format );
	} else {
		/*
		 * Timestamp with offset is typically produced by a UTC `strtotime()` call on an input without timezone.
		 * This is the best attempt to reverse that operation into a local time to use.
		 */
		$local_time = gmdate( 'Y-m-d H:i:s', $timestamp );
		$timezone   = wp_timezone();
		$datetime   = date_create( $local_time, $timezone );
		$date       = wp_date( $format, $datetime->getTimestamp(), $timezone );
	}

	/**
	 * Filters the date formatted based on the locale.
	 *
	 * @since 2.8.0
	 *
	 * @param string $date      Formatted date string.
	 * @param string $format    Format to display the date.
	 * @param int    $timestamp A sum of Unix timestamp and timezone offset in seconds.
	 *                          Might be without offset if input omitted timestamp but requested GMT.
	 * @param bool   $gmt       Whether to use GMT timezone. Only applies if timestamp was not provided.
	 *                          Default false.
	 */
	$date = apply_filters( 'date_i18n', $date, $format, $timestamp, $gmt );

	return $date;
}