group function not allowed in where clause?

Hi there.

I try this on a Oracle 10g on Windows2000 Server:

select pl.probeid, max(pl.logdate) as lastcheck, (sysdate - max (pl.logdate)) * 24*60*60 as timespan, p.interval from tblprobeloghttp pl inner join tblprobehttp p on pl.PROBEID = p.PROBEID
group by pl.probeid, p.interval

this works perfectly, but I want only those where timespan is larger than interval, so I tried to add a

where timespan > p.interval

but this gives me an

ORA-00904 "TIMESPAN": invalid identifier

when I try:
where ((sysdate - max(pl.logdate))*24"60"60) > interval I get an:

ORA-00934: group function is not allowed here.

How can I accomplish this? Is it only possible with PL/SQL?

Thanks for any help

-- 
mfg
Marc