I'm stuck. Using Excel 2002, Win XP Pro

Here's a two-column data table and one column is where I enter a daily
number. The next column shows the difference between the present day and
the day before as a positive number.

Here's the file:
http://home.alltel.net/sayres5762/differencechart.xls

I'm using =IF(C80,C7-C8,"") to calculate the difference and display a
blank if there's no data.

The problem is when I must skip a day or two. I want to be able to leave
empty cells in Column C and have it not affect the Difference column or the
Total Difference. And if the Data entry is blank, I want the Difference to
display a blank, as well.

On the table enter a number.. say "80" on January 8 and you'll see what I
mean.

I've learned a few simple formulas but still have a way to go. Any help
would be appreciated.

Steve