[WP] Calendar Widget force prev/next month

function get_calendar_filter( $input ) {
    global $year, $monthnum, $m, $wp_locale;
    $output = $input;
    // 現在の年月を取得
    $ts = current_time('timestamp');
    $currentyear  = (int) gmdate('Y', $ts);
    $currentmonth = gmdate('m', $ts);
    // グローバル変数から現在のページの年月を取得
    if ( $year && $monthnum ) {
        $thisyear  = (int) $year;
        $thismonth = (int) sprintf( '%02s', $monthnum );
    } elseif ( isset($m) && is_numeric($m) && strlen($m) > 3 ) {
        $thisyear = (int) substr($m, 0, 4);
        if ( strlen($m) < 6 )
            $thismonth = 1;
        else
            $thismonth = (int) substr($m, 4, 2);
    }
    // 年月のチェック
    if ( $thisyear > 1970 && $thismonth > 0 && $thismonth < 13 ) {
        $thisyear  = $thisyear;
        $thismonth = sprintf( '%02s', $thismonth );
    } else { // 不正なら現在の年月を使用
        $thisyear  = $currentyear;
        $thismonth = $currentmonth;
    }
    // 前月・翌月を取得
    if ( $thismonth === '01' ) {
        $prev = array($thisyear - 1, 12);
        $next = array($thisyear,      2);
    } elseif ( $thismonth === '12' ) {
        $prev = array($thisyear,     11);
        $next = array($thisyear + 1,  1);
    } else {
        $prev = array($thisyear, (int) $thismonth - 1 );
        $next = array($thisyear, (int) $thismonth + 1 );
    }
    // 前月を書き換え
    if ( ($currentyear * 100 + $currentmonth) >= ($prev[0] * 100 + $prev[1]) ) { // 未来でなければ
        $prev_pattern = '/<td colspan="3" id="prev"[^>]*>.+?<\/td>/';
        $prev_replace = '<td colspan="3" id="prev"><a href="'.get_month_link($prev[0], $prev[1]).'">&laquo; '
            .$wp_locale->get_month_abbrev($wp_locale->get_month($prev[1])).'</a></td>';
        $output = preg_replace( $prev_pattern, $prev_replace, $output );
    }
    // 翌月を書き換え
    if ( ($currentyear * 100 + $currentmonth) >= ($next[0] * 100 + $next[1]) ) { // 未来でなければ
        $next_pattern = '/<td colspan="3" id="next"[^>]*>.+?<\/td>/';
        $next_replace = '<td colspan="3" id="next"><a href="'.get_month_link($next[0], $next[1]).'">'
            .$wp_locale->get_month_abbrev($wp_locale->get_month($next[1])).' &raquo;</a></td>';
        $output = preg_replace( $next_pattern, $next_replace, $output );
    }
    return $output;
}
add_filter( 'get_calendar', 'get_calendar_filter' );