If these are date fields with no time component (or if the time component from both is 12:00:00 AM), you can just use {table.date2} - {table.date1} instead of using DateDiff. If there is a time component you can do something like this:
({table.date2} - {table.date1) \ 1
Note that this uses backslash '\' which is the modulus operator instead of slash '/' which is the division operator. Modulus gives you the whole number part of the equation. You would want to use this instead of Round because Round will round up the number if there is more than 12 hours difference.
-Dell