How to prevent negative values from being displayed

Mark beat me to it but since I already took the time to test it..

You can do it very simply by just adding max to your formula. This will show a 0 for anything in the future and the actual days taken for anything in the past. You can also get into doing this for workdays as well but you didn't mention needing that.

var date EndDate = Today();
max(0,ToDays($EndDate - [Date of Release]))

You could also do it in a way that didn't show 0 or a blank but instead showed text of some sort, like "Not Started" "Work Pending" or whatever you wanted. It would have to be a text field instead of numeric though, which may or may not work for you.