I have a spreadsheet with sub-totals which calculate the columns at the
bottom - like below.

April 1.00
May 2.00
August 1.50
September 3.00
October 5.00
Etc.
Sub-totals:- 12.50

The information for September and October is hidden by using conditional
formatting and will only be visible at the end of the respective month.
Unfortunately though, the information is still showing in the sub-total. In
the example above, I only want the sub-total to show 4.50, then at the end of
September it should be 7.50.

I don't want to have to keep chaning the formula every month to include the
extra line and believe there must be a way to do this.

Thanks in advance.