Need help with calculating date

Hi..

I need help calculating a date value..  This is the script:

select

 target_name,

 target_type,

 sum(trunc((nvl(end_timestamp,sysdate)-start_timestamp)*24)) total_hours

FROM mgmt$Availability_history

where target_name = 'ORCL'

group by target_name,availability_status, target_type

/

 

This scripts operates against the OEM view mgmt$availability_history.  The results of this query are in total hours for a six month period.  I need to limit the time period to 30 days of values.  How would I best accomplish this?

Thanks..

NS