try something like this..
$time=time(); that goes to database
then..
for checking before post
//select results
//check username..
$result=mysql_query("SELECT * FROM shout WHERE username='$username' ORDER BY time DESC LIMIT 1");
$r=mysql_fetch_array($result);
//then
$timeout=$time-60;
if($r[time] < $timeout);
{
//cant post
}
else
{
//post
}