[syslog-ng] Variable issue with syslog-ng

I am trying to setup syslog-ng to use the DAY and MONTH variable on the source file input. What am I doing wrong? source s_file { file("/psfs_logs/APPSRV_${MONTH}${DAY}.LOG" flags(no-parse)); }; destination d_messages{ udp("10.13.33.11"); }; log { source(s_file); destination(d_messages); }; Any help would be greatly appreciated...