$since_navi = mktime(0, 0, 0, date("m"), date("d")+7, date("Y"));
$termine_navi2 = mysql_query("SELECT COUNT(date) FROM termine WHERE stamp <= " . $since_navi);
$termine_navi2 = mysql_fetch_row($termine_navi2);
was passiert denn, wenn Du die Abfrage so änderst?:
$termine_navi2 = mysql_query("SELECT COUNT(date) FROM termine WHERE stamp <= '" . $since_navi ."'");
Gruß
Reiner