I am adding monthly data and I need to get 12 months worth. Is there a
formula that will result in an "error" if I have a zero in one of the cells?
Can this be conditionally formatted to pick up the missing data?

Column "A" has data that is input. Column "B" needs to sum 12 cells from
Column "A", example =sum(a1:a12). How to format when column "A" has yet to
get data.