Humm - not simple - and probably impossible if you are using Open Office documents as certificates.

One thing you might consider (assuming that you are running 1.9 and using its invoice template) is to tick the 'Reminders on invoice' checkbox.  As it says on the help screen at https://openvpms.org/documentation/csh/1.9/admin/template/letterhead/edit :

Reminders on Invoice - check this box to display on invoices any patient reminders due in the next 12 months.  A maximum of 4 lines are shown with reminders for products of the same product type on the same day for multiple patients combined on the one line.

Next trick: tweak the certificate document that you are using to either:

a) calculate the next due date by adding the appropriate interval to the startTime  (using the date:add(dateTime, period) function - see https://openvpms.org/documentation/csh/1.9/reference/jxpath

b) add a user field that requests the next due date as an input and display this on the certificate

Regards, Tim G