Hi pquintan,
It looks like you use the function DateDiff("s",db1.open_time,db1.close_time )at the SQL level.
This function doesn't exist in the BO reporter.
You will maybe find a solution on a "SQL" forum. Search SQL on the forum list and choose the one in relation with your database (Sybase, Oracle, IBM,...)
My thinking about that is that you can be happy that your database gives your this kind of function: DateDiff(,,,).
Hope it will help.
GKar