Old Invoice

You can show original date of invoice in books and show it in next month 3B returns.
But tell the seller to include the invoice in his GSTR1 returns.

Yes, this will be wrong practice in GST , so convince your supplier to upload the invoice in Dec 17 & you aslo book the invoice in Dec 17 & take the input in current month.

But I assume that your supplier will not agree to do so , as he will be not ready to deposit Interest for the same so, if such parctice done once while it's ok ,. else avoid such practices on reqular basis .
Do as the your supplier suggest.

Yes, I too agree with Pankaj sir views....