Is it possible to change the billing date?

We have the day to pay on the 30th or 31st of each month. We want to know if it is possible to change the date to the 15th of each month.

Unfortunately, it's not possible to change the billing date.  If you purchased the subscription on November 30th, then it will renew on the same date.

You can offer a solution to this "problem"?

You can upgrade your account on December 15th to change the billing date. The unused days for your current subscription will be automatically applied to the next charge so you'll only pay for the difference.