Automated Estimate at Completion

Hi Everyone,
This may be a VB answer but I thought I'd try here first.

I am trying to find out the Estimate at Completion - I'd like to sum up all of the previous months Actuals and then the forecast for the months remaining.
If the spreadsheet has to be set up differently, please let me know.
Currently I have 25 months and the data like this:
Jan 16 Forecast Jan 16 Actual...

Is there a formula that will update the sum on the first of each month...?