In Zoho Books, I need to assign the total hours to be billed for the project.
In my case, I need to assign 20 hours (for example) for a project and bill the customer every monthly when the last day is reached. In addiction it is important to accumulate the hours when the customer will not use the hours either by request or support for instance for the next month.
Like it,
Client, Contoso has 20 hours per monthly in contract with us it costs to him 500$ per month.
They had used only 10 hours in February in my concern, he has 10 hours remaining to use in March. As a matter of fact, In Zoho Books, need to be billed the project normally and showing in the portal for the customer that he has 30 hours in total, because he had use only 10 hours in the last month.

Feel free to contact me in case of doubt.
Best Regards
Jonas