[syslog-ng]Syslog-ng log file rollover question

Hi all, I am wondering whether anyone here knows how to config syslog-ng to perform log file rollover. I was told that I can roll over a log file in syslog-ng by configuring syslog-ng in a format which includes time related macroes, such as $MONTH, $DAY, $HOUR, $MIN. For example destination snort { file("/var/snort/snort-$MONTH$DAY$HOUR$MIN"); }; In this case, the log file should roll over to a new file every 1 minute. However, I found it doesn't work and syslog-ng keeps appending its received syslog message into a log file, for example snort-08091208. Does anyone know how to do it or figure out what I have done wrong? Or does syslog-ng support the log file rollover? Thanks! Xiaodong