Copy and paste it into your Immediate window and keep changing the value of DateVariable. It works.
I think that dates are stored in Access as numbers, with the whole number(number to the left of the decimal) as the date, and the number to the right of the decimal as the time. In Excel it's definitely like that, so I assume in Access as well.
Someone wrote a very similar problem in this thread a few days ago. The link in there provides a nice and easy way(s) to calculate something similar to what you want. I advise you read it (it's fairly easy to follow) and see if you can understand the logic that is involved. Post any questions back here if you want
Next comes the question of where (or how) do you want this calculation to be displayed...