mirc does recognize it's a leap year, you can verify that with
//echo -ag $duration($calc($ctime(31/12/2007) - $ctime(01/01/2007)))
You just have the wrong formula; essentially you're leaving the last day of December out of your calculations. Try this:
$duration($calc($ctime(01/01/2009) - $ctime(01/01/2008)))
