I'm not sure why it would be giving negative numbers unless some of your dates are actually greater than today. You may be seeing the wrong answer if the calculation is using the sum aggregation. To change this, you can either add avg to the front of the calculation, or right click on it on the view, and select Measure-->Average.
Hope this helps!
It worked as you rightly pointed out. I had to change it to AVG.
Thanks again for the prompt help.