17 October 2017
Self invoices issued by you under RCM are taxable invoices. Therefore, you can take credit of tax paid as long as these invoices are valid. In my view, you can take the credit and utilize the same for payment of GST against outward supplies.
31 October 2026
Missed RCM liability can be declared in a later GSTR-3B, but it must be paid in cash (RCM tax cannot be set off from the ITC ledger) with 18% interest u/s 50(1) from the original due date; ITC of it is then available subject to the Sec 16(4) time limit of 30 November following the FY of the self-invoice.