So vergleichen Sie den Zeitstempel in der Where-Klausel

SELECT users.username, users.id, count(tahminler.tahmin)as tahmins_no 
FROM users 
LEFT JOIN tahminler ON users.id = tahminler.user_id  
where year(timestamp) = 2013 and month(timestamp) = 9
GROUP BY users.id 
having count(tahminler.tahmin) > 0

Damit es mit Indizes funktioniert, können Sie

SELECT users.username, users.id, count(tahminler.tahmin)as tahmins_no 
FROM users 
LEFT JOIN tahminler ON users.id = tahminler.user_id  
where timestamp >= '2013-09-01' and timestamp < '2013-10-01'
GROUP BY users.id 
having count(tahminler.tahmin) > 0