I had this problem the other week, when I was trying this statement (=DateDiff("n",[Equipment Stop Time],[Equipment Start Time]) in an unbound text called Actual Down Time to a field name in a table called Actual Down Time without any success.
So I set up a bound text box [Actual Down Time]
Then I used an macro.
I set the macro up like this: Action = SetValue , Item = [Forms]![FrmDowntime]![Actual Dowm Time]
Expression = =DateDiff("n",[Equipment Stop Time],[Equipment Start Time])
Then I set the form up by clicking on the properties for [Equipment Stop Time] and [Equipment Start Time]) and setting the after update to the macro and this is working fine.
So My calculation's for working out downtime between two times is correct an it is sending the information to the table.
I hope this little bit of information has pointed you in the right direction.
HL