Something of this form:

=IF(A1=0,Action1,Action2)

Action1 is "last cumulative value" (?)
Action2 is "last cumulative value" - A1

Or if you want the cell to blank for future dates:

=if(A1=0,"",B1-A1)

HTH

"patt" wrote:

Hi. I have a cumulative budget worksheet where I add weekly numbers and will
show a profit/loss for each week as I go. It worked fine last year! but
whatever I did now, it picks up the blank cell (weeks I haven't come to yet)
as zero and so automatically shows all those weeks as losses all the way down
the page.

How can I get it to not pick up a cell as zero? To not subtract that cell
unless there is a number in it?

Thanks for your help.