Selected Answer
Graham
You can use Excel's DAY function (which returns the day part of a date) within an IF statement.
In the attached, revised file I've put this in cell W2:
=IFERROR(IF(DAY(A2)=12, 4494.31,""),"")
and fill (or copy) down and the value in bold above will appear on the 12th of whatever month (row 13 in your file).
REVISION:
To create the balance in column U, I put your opening balance of $600 (from 30 November) in sheet DECEMBER22, cell T1. This is the Opening Balance for entire the workbook, in other sheets,T1 is the balance from the end of the previous month (cell U33 of the previous tab).
In cell U2, I then added any incomes that day to that balance, less any bills to give the balance at the end of that day:
=T1 + SUM(W2:Y2) - SUM(B2:R2)
The formula for cells U3 and down is like:
=U2+SUM(W3:Y3)-SUM(B3:R3)
so the day's transactions affect the balance from the previous day.
(Totals line) Balance cell U33 has this formula:
=T1+SUM(V33:W33)-SUM(B33:R33)
which is the opening balance for the sheet less the sum of incomes and bills.
Hope this helps. If so, please remember to mark this Answer as Selected