Can gst on previous months sales invoice which was missed be set off from itc of next month

We have received advance (with 18% gst) for services (works contract) in May 2020. Services will be rendered in June, 2020.

However, we forgot to show advance received on services as taxable supply in GSTR3B of May, 2020 and thus did not pay gst on time. Further, there was no ITC available in May, 2020.

We are now showing it as taxable supply in June, 2020 GSTR3B and will have to pay interest @ 18% per annum for late payment of GST.

However, there is ITC available in return for June, 2020 for purchases made during June, 2020.

1. Can we offset gst payable (may output liability shown late in june return) with june ITC.

2. If yes, whether interest will be paid on gross amount (gst on total advance) or net amount (paid in cash after offset).