Timestamp truncation in MonetDB

Hi, I'm trying to truncate timestamp value to hour and/or minute precision. I combined cast and extract functions as: Problem: Truncate '2013-08-12 23:59:59' to '2013-08-12 23:00:00' select cast(timestamp '2013-08-12 23:59:59' as date)||' '||extract(hour from timestamp '2013-08-12 23:59:59')||':00:00'; Output: 2013-08-12 23:00:00 But this is expensive because of repeated function calls. Is there any faster and built-in function? Regards, Ravi