strtotime will automatically use the current unix timestamp to base your string annotation off of.
Just do:
$date = strtotime("+7 day");
echo date('M d, Y', $date);
Added Info For Future Visitors: If you need to pass a timestamp to the function, the below will work.
This will calculate 7 days from yesterday:
$timestamp = time()-86400;
$date = strtotime("+7 day", $timestamp);
echo date('M d, Y', $date);