[Bro] generating bro report from packet capture

[mel]

Hi,

I want to generate bro reports from tcpdump packet capture files. Those 
files were captured from different networks. So:

bro -r file.pcap will generate generate log files:

alarm.log
conn.log
notice.log
weird.log

When I run site-report.pl, I get

No connection data found for the time period specified.
Unable to create a report.

What does this error mean? Note that the packet capture files maybe 
several days old.

--mel