I took a quick look at this – one possible quick fix could be adding the integer function to your formula:
Int(Finish1-Finish)
It returns the integer portion of a number. I call this a quick fix because I think there are several ways you can have a decimal in this field and you might want to investigate further. If there is a difference in the working times between the two different projects, this might be a cause. You can show the time along with the date to see if one finish is at 5:00 and the other is at a different time (options/general/date format).
