[AccessD] A2K: Time diff calcs

Hello all
I want to calculate then insert into a table the duration of a users logon
in HOURS and MINUTES
In the logon table the default value for the logon field is Now() and is
captured at...You guessed at logon. I then run a query that updates the
Logoff field to Now() once the user answers YES to loggin off. The theory
being there will be a difference even if it is only 5 or 10 seconds. Unless
some clown plays with the clock after loggin on --- but forget all that :-))

Both logon and logoff fields data types in the Logon Table are General Date
(01/01/2004 15:45:25PM)

So a user logs on at say...14/09/2004 10:08:45 PM and then logs off at
say...14/09/2004 10:10:55 PM

The time logged on = 2 mins and 10 secs but how do I calculate that?

I have udsed DateDiff but with no success...
DateDiff("Short Time",[LogonTimeAndDate],Now())
DateDiff("mm",[LogonTimeAndDate],Now())
DateDiff("nn",[LogonTimeAndDate],Now()) etc etc

Many thanks in advance

Darren