Mistake in formula of pro rata billing

[knoparik]

I suppose there is a mistake in formula of pro rata billing in invoices and in orders.

Question: May you expose certain formula of pro rata billing?
How you calculate days in period? How many days in your calendar? etc.

I think, an error is inside quantity of days for period.
Sometime appears one additional day in calculations. It leads to wrong total price of invoice and order.

For example, if I order on October some products & services for 6 or 12 months, the mistake does not exist.
If I order some products & services for 1 or 3 months, the mistake exists in formula.

First time, when I got this mistake, it was September 2018.
On September the mistake appeared only for 3 and 6 months period orders, but does not exist for 1 or 12 months periods.