ReportBuilder and Timestamp math

I am evaluating Report builder. One particular problem I have is to get
the difference between two time stamps (e.g.received and despatched)
and also an average for the total report.

So for each record printed I show the time difference and at the bottom
of the report I show the average time difference for the entire report.
I have wrestled with QR2 and various queries but still do not get the
results I want. The RB dbcalc fields seem to be less functional than
QR2 but I suspect I am missing something.

Pat Bell

Using Virtual Access