Excel 2007 : need formula to work only if the cell is populated

I have 1 column that contains numbers from a counter. The next column will subtract yesterday's number from today's number giving a difference. Even though there is a cell for each day. The counters are not checked every day. Therefore some cells are blank. So I need the formula to then look above to find the next previous populated cell. Having a hard time figuring this out. Any help will greatly appreciated. I have attached a sheet.counters.xlsx


Thanks in advance