I have a ramdon circulater that changes a piece of text once a day, and I use the following code to make sure the see is unique enough.
$tm = time();
$today = mktime(0, 0, 0, (int)date("n", $tm), (int)date("j", $tm), (int)date("Y", $tm));
srand($today / pi());
The pi works wonders for the whole thing and it works like a charm. Any other big decimal number will do as well, but pi is the most common "big" number.