How do I calculate % of task's completion given start date, end date, and TODAY()

Question

I have a gant with start date, end date, and % complete columns. By manually entering a number in the % column the bar representing the task gets shaded. What I want to do is instead of representing % completed, I want to show how much time is left before the end date from today.

      Start        End       % Time remaining from TODAY()
i.e. 12/01/2014   03/15/2015   (End date has not yet occurred)
     12/29/2014   12/29/2014   (Task was started and finished this day)

Answer 1

Assuming your end date is in column B:

=IF(TODAY()>=B2,"Done",CONCATENATE(B2-TODAY(),""))

This will show you the number of days remaining. If you want the percentage of time spent, use

=IF(TODAY()>=B2,"Done",MAX((TODAY()-A2)/MAX(B2-A2,1),0))

and format the cell as a percentage.

Answer 2

Here is a bit more succinct option it will show the percent completion as of the end of the current day.

=(MIN(TODAY(),B2)-A2+1)/(B2-A2+1)