Auto calculate by date

Hi all, I will try to keep this brief.
A little info first...
I have 12 worksheets January to December Each month contains my house hold bills for that month. They are listed by date 1 to 31 dependant on payment date (around 14 payments each month).
Each month I find myself checking my bank balance then opening my excel worksheets and calculating what free cash I have at any one point in the month, this is done with a calculator and paper.

My question...
Is there a formula or script I can use in one cell, within each worksheet. Whereby I can input my bank balance on for example the 12th day of a month and the calculation can be done automatically, just by taking off all the payments which have occurred on or before the 12th. But could the cell automatically know the current date so I can just make one input to get the desired result.

Hope this is not to complex
Many thanks

Mark

If each payment is shown with payment day (just a number 1 to 31 not a date) in A2:A20 and corresponding amount in B2:B20 then try inputting current balance in D1 then this formula will deduct all future payments for that month from that amount

=D1-SUMIF(A2:A20,">"&DAY(TODAY()),B2:B20)

Many thanks Daddylonglegs. Excellent job, the Formula has done exactly what I wanted it to do. Thanks again.