Some subscription options are based on the table `tblhosting` (or hosting if it hasn't a prefix).
Next_due_date & next_invoice_date are some of the columns that show you some info about the subscription status.
Based on that, you can search into whmcs code (on some free/unencoded versions eg. 7) to find where that columns are used and how to manage data.
Hope someone find this a little helpful. :)