Follow these steps to get subtotal of invoices by datewise (age):
1- Calculate first the number of days only by using DAY function then copy and paste the formula from D2 cell to D10 cell.
2- Now type SUMIF function in a blank cell where you want total against datewise. (e.g. cell H6)
3- Select range of cells that you want to evaluate against age criteria.
4- Select criteria of “<=20” which means sort and sum up all invoices which are equal to or greater than 20 days.
5- Select actual cells to add against selected criteria.
6- After pressing Enter button, Excel will sum up all invoices which are equal to or greater than 20 days.