\WP_Stager_Integration\Calendar::get_year_number(int $timestamp = null): int

Full Code

/**
 * Get a formatted year number. Takes the site's timezone setting into account.
 *
 * Returns the ISO 8601 week-numbering year. This has the same value as PHP's <code>date('Y')</code>, except that if the ISO week number (W) belongs to the previous or next year, that year is used instead.
 *
 * @param int    $timestamp  (Optional) The Unix timestamp to get the number for. Defaults to current server time.
 *
 * @return int  The year number
 */
public static function get_year_number( $timestamp = null ) {

    // Type casting
    $timestamp = (int) ($timestamp ?: time());

    return (int) wp_date( 'o', $timestamp ); // ISO 8601 week-numbering year. This has the same value as Y, except that if the ISO week number (W) belongs to the previous or next year, that year is used instead.
}