Automatically update SUM range for multiple cells

Hi everyone,

Hope you're all doing well.

Bit of background. I have a large spreadsheet, which I have simplified, that I input all my costs for the relevant month in the 'Budget Tracker' tab, which then links to the 'Cashflow' tab.
In the cashflow tab there are two columns that either sum up the 'spend to date' or the 'cost to complete'. 'Spend to date' calculates all the formulated data that has been inputted into the 'Budget Tracker' tab and cost to complete calculates the remaining cells in the range that have been manually inputted.

I have been asked to whatever way possible create a formula that does the following:

When we move to the next month and input costs into the 'Budget Tracker', June for this instance, we want the SUM range for all 'spend to date' cells to include June's figures, whilst also reducing the SUM range for all 'cost to complete' cells to then be from July onwards, at the same time.

At the moment we update the first equation for both 'spend to date' and 'cost to complete' and drag it down, however, as the spreadsheet is so big, we would like to automate it so it updates automatically for each month.

Really sorry if this doesn't make sense.

I appreciate all your assistance.

Thank you.