Today’s XKCD comic is an inspiration. Click on the image to see the full size cartoon and then come back here to wallow in nerdiness.
Of course, I had to see whether the formula would actually work so, I plugged it into Excel and it looks like this:
Hopefully it is obvious that cell A2 contains the current date and cell B2 is the percentage completion expressed as a fraction.
It works, after a fashion, although the date acceleration is nothing like that suggested by the cartoon. More amusingly, though, is the fact that the Excel date function breaks when I exceed 72.0857% completed. This is obviously because nothing can ever be more than 72.0857% complete when Microsoft are involved.