DateTime::diff(DateTime [， bool absolute = false])中的绝对参数做什么?

下面是示例代码

$c = new DateTime();
$o = clone $c;
$o->modify('-60 days');
$diff = $c->diff($o);
$diff2 = $c->diff($o, TRUE);
var_dump($diff, $diff2);

输出

object(DateInterval)#3 (8) {
  ["y"]=> int(0), ["m"]=> int(1), ["d"]=> int(29), ["h"]=> int(0), ["i"]=> int(0),
  ["s"]=> int(0), ["invert"]=> int(1), ["days"]=> int(60)
}
object(DateInterval)#4 (8) {
  ["y"]=> int(0), ["m"]=> int(1), ["d"]=> int(29), ["h"]=> int(0), ["i"]=> int(0),
  ["s"]=> int(0), ["invert"]=> int(0), ["days"]=> int(60)
}

如我所见，只有"invert"属性改变了。这是什么意思?

Absolute parameter determines whether to return absolute difference.