This is the closest I have found to an explanation of what I'm asking, but as it states it does not work for current month.
https://www.odoo.com/forum/help-1/accounting-how-do-i-set-a-due-date-on-every-9th-of-the-month-249582
I need the closest 15th, so if the invoicing date is the 3rd of the month, that same month on the 15th the payment would be expected, not on the next month.
Any way to do this without customisations? If not, any community module for that?