As per the Provision of SECTION 16(4) of CGST Act 2017, A registered person shall not be entitled to take input tax credit in respect of any invoice or debit note for supply of goods or services or both after the due date of furnishing of the return under Section 39 for the month of September following the end of financial year to which such invoice or invoice relating to such debit note pertains or furnishing of the relevant annual return, whichever is earlier.
Since the Annual Return ( Form GSTR 9 ) is not available yet in the common portal, hence Last date to claim ITC for FY 2017-18 is 20th October 2018.