Comparing events from 2 dates to detect new events

[Alon Agmon]

Hi,
were using NMAP via scripted input to track live hosts on the network
im getting events formated using sed like:
Fri Apr 9 16:11:50 IDT 2010 host=192.168.0.157 message= is up 
Fri Apr 9 16:11:50 IDT 2010 host=192.168.0.158 message= is up
Fri Apr 9 16:11:50 IDT 2010 host=192.168.0.185 message= is up

now how can i check if there are new hosts today compare to last week ?

i tried using "diff" on a scenario when 192.168.0.185 was down in searchA and now is up in searchB:
source="nmap-hosts" | diff [search date_minute=43] [search date_minute=31] attribute=ip

but that just gave me: 
-192.168.0.186
+192.168.0.185

while actually nothing happened with *.186 
and the actual ip field of the event in the result was 186 (??)

any idea on how to handle this ..??
we are really stuck on this, since its an important requirement
that we can freeze status of live hosts, ports, risks
and than run a schedule search to detect new events ...

Alon.