SELECT * FROM table1 WHERE timestamp >= DATEADD(HOUR,18,CONVERT(VARCHAR(10), GETDATE(),110)) AND timestamp <= DATEADD(HOUR,6,CONVERT(VARCHAR(10), GETDATE()+1,110))
SELECT * FROM table1 WHERE timestamp >= DATEADD(HOUR,18,CONVERT(VARCHAR(10), GETDATE(),110)) AND timestamp <= DATEADD(HOUR,6,CONVERT(VARCHAR(10), GETDATE()+1,110))