The Due date of filing GST return for September 2018 ( FY 2018-19) is 20th October 2018 and the Date of filing GST Annual Return for FY 2017-18 is 31st December 2018, whichever is earlier.
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.
Furnishing month of Return is 11th October'2018 - GSTR-1 and 20th October '2018- GSTR-3B.
ITC can be claimed only within one year from the date of issue of tax invoice relating to supply. In any other case, the last date to claim ITC is earlier of the two below:
Before the filing of valid return for the month of September following the end of FY to which such invoice is related, or Before the filing of the annual return
Note: As per section 44, the last date for filing of annual return is 31st December following the end of the financial year.