Adding cells depending on a date

[Keith]

I have a spreadsheet with three sets of columns for each month going along
the top. I have actual figures, worst case and best case, with each column
totalled at the bottom.

At the far right (after December) I want a running total of all the actual
totals based on a cell containing a month. So if the month cell is January
the total cell will just equal the total cell for January, but if it's March
then it will equal the sum of the January, February and March totals.

Is there a simple way to do this without a huge formula with lots of IF
statements?

 

[Gary''s Student]

1. make sure that each month cell will contain zero or a blank until the data
for that month has been entered.

2. then make the sum of ALL months, future months will contribute zero, but
that's o.k.

 

[Keith]

Thats no use. They want to input a whole years data then simply get a total
at any point in the year by simply inputing a month.

 

[Herbert Seidenberg]

Excel 2007
Pivot Table
with update macro
http://www.savefile.com/files/1905705