Thanks for the help in advance I'm stuck.

I have an invoice table.
The table has a "number of payments" column
and a "first payment date" column.

I need that while the cells in these columns are 
entered with data, a list of monthly payments 
will be created where each date contains the 
(day, month and year + month) from the day 
of the date in the "first payment date" column, 
multiplied by the number of payments in the 
"first payment date" column.

Next to each payment date line there should 
be an alert box indicating whether the 
payment was paid, paid late, or not paid, 
and send an alert for a payment that was 
not received on time.
Another column that shows the number 
of days that have passed since the date 
of the unpaid payment.

I have a "Payments" sub-table under the 
Invoices table.