I had a similar problem converting YYYYxxx (such as 2002001 for jan 1, 2002) where xxx was the day of the year and here is the function I used to convert it into a unix timestamp....
//INPUT $string YYYYxxx
//Output = formatted unix timestamp
//mm/dd/YYYY
function rx30_to_human($rxdate)
{
$year = substr($rxdate, 0, 4);
$day = substr($rxdate, -3);
$timestamp=mktime(0,0,0,1,$day,$year);
$result =date('m/d/Y',$timestamp);
return $result;
}