note 11747 added to microtime

I am working on a NT station (5.5, Whistler, XP) and I noticed something REALLY strange! When I get the time at the start of a PHP-file and at the end, and I do $et - $bt, where $et is the endtime and $bt the begintime and the msec part is put behind the sec part, that it sometimes returned a value below zero. How the f*** is this possible? The end time can never be before the begin time! Even when I do an echo "$bt : $et", I can see with my own eyes that the end time sometimes is behind the begin time! Anybody any idea what can causing this? Just NT, or? http://www.php.net/manual/en/function.microtime.php