As per info at http://www.amember.com/docs/How_to_set_up_custom_invoice_numbering, I've modified to show year and month and day with 'id' pre-pending the numbering. $invoice->public_id = date('Y_F_d') . '_id'. ++$last_invoice_num; It seems to work ok, but then reverts to the previous format of a 5-digit code minus any date info. This has happened after changing setup to 'put PayPal before aMember'... I don't know if that's the cause, but I can't see why it might be. And, oddly, if I replace 'Y_F_d' with 'Y_m_' and then revert to 'Y_F_d' it seems to work ok again. This makes no sense to me.