The name of the log is actually a magic cookie string, where cookies representing time are expanded using strftime(3). For example, a log name of /var/log/ncftpd/sess.%y%m%d would create daily logs named /var/log/ncftpd/sess.971009, /var/log/ncftpd/sess.971010, etc.
You can also use the cookie "@SETNAME@" which expands to the value of the set-name variable you used for this domain.
The format of the session logs is also documented.
Examples:
