How to calculate dates and ongoing revenue

Hi everyone, as part of my offering I have a membership community that people pay monthly for. This is hosted on Squarespace. In my Zoho CRM I have added a field for the date a person becomes in active. This means there is a "closing date" which is the date they joined and then there is the end date which is the date they cancelled their membership. Is there a way to calculate how many months they were active in the community? This is an important stat for me to have.

Second, is there a way for the value to go up by 25 (membership fee) each month automatically, or do I need to update this myself?