Payment terms collect at a day in a month


I need help in making my payment terms collect payment at a day in a month

Such as day 15 of each month

If i do it using the normal payment terms i have to set number of days after invoice or invoice end month, the result is always different between invoice and another

I need it to be like day 15 each month

If I am creating two invoices on 10th April and 20th April, what will be the due date of these invoices? Is it April 15 and May 15 respectively?

Yes Thats what i mean

Any help