Gst input.

Yes you can claim earlier of filing Annual Return or 20th October'2018, So you can claim in current returns and make adjustments accordingly in Annual Return

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.