First things first: Are the fields in question date/time data types at the
table level?

Next what do you mean by "Each record has a parameter in it"? Do you mean
that each record has a value in it OR this is a parameter query that prompts
you for dates when run?

Further by "Everything keeps coming back with no results" do you mean that
no records are returned or just nothing in the calculated field?

Re: > Everything keeps coming back with no results.  
The above statement offers us no useful information.

Give us an example of the values in the 2 fields. What result should
be shown? What value is being shown? An #Error? Nothing? An incorrect
value? What?

Did you ever read VBA help on the DateDiff function?
The arguments are specific. You need to separate the 2 values with a
comma, not instruct Access to subtract one from the other. Access
knows that.

TimeDifference:DateDiff ("d", [Received Date Time],[Opened Date Time])

will determine, in days,  the difference between the 2 values.
Note: the above will return a value of 1 day even if the [Received
Date Time] is 11/6/2007 11:59:59 PM and the [Opened DateTime] is
11/7/2007 12:00:01 AM, just 2 seconds later, because the date boundary
has been changed.

You use a - while a coma is needed.  Use

DateDiff ("d", [Received Date Time] , [Opened Date Time])

instead of your initial syntax:

DateDiff ("d", [Received Date Time] - [Opened Date Time])

Vanderghast, Access MVP