Source

File: wp-includes/rest-api.php

function rest_get_date_with_gmt( $date, $is_utc = false ) {
	/*
	 * Whether or not the original date actually has a timezone string
	 * changes the way we need to do timezone conversion.
	 * Store this info before parsing the date, and use it later.
	 */
	$has_timezone = preg_match( '#(Z|[+-]\d{2}(:\d{2})?)$#', $date );

	$date = rest_parse_date( $date );

	if ( empty( $date ) ) {
		return null;
	}

	/*
	 * At this point $date could either be a local date (if we were passed
	 * a *local* date without a timezone offset) or a UTC date (otherwise).
	 * Timezone conversion needs to be handled differently between these two cases.
	 */
	if ( ! $is_utc && ! $has_timezone ) {
		$local = gmdate( 'Y-m-d H:i:s', $date );
		$utc   = get_gmt_from_date( $local );
	} else {
		$utc   = gmdate( 'Y-m-d H:i:s', $date );
		$local = get_date_from_gmt( $utc );
	}

	return array( $local, $utc );
}