Formula Help (new to Spreadsheet)
 

I am trying to calulate a field that represents money due in 30 days.

I need a formula that will essentially look at... if there is a balance due
that is within the next 30 days or today plus 29 days it will sum the
balances due within that fixed period.

Obviously this is a dynamic number, that cahanges daily, weekly etc. Can
anyone help me with this?