Calculating Days remaining

In one cell I am calculating days remaining.  I would like for the days remaining to stop when there are no days left and/or if the progress fields equals zero.  I am trying to use the IF command but can't figure out how to get it to work.  So far, I am calculating days remaining with the following formula:

=H11-TODAY()

What do I add to this formula so that it stops calculating when there are zero days left?