21 November 2018
No you cant claim this input after filling of Sep. 2018 returns if you are regular taxpayer. It may, however, be noted that the Government has extended the last date for furnishing of return in FORM GSTR-3B for the month of September, 2018 for certain taxpayers who have been recently migrated from erstwhile tax regime to GST regime vide notification No. 47/2018- Central Tax dated 10th September, 2018. For such taxpayers, the extended date i.e. 31st December, 2018 or the date of filing of annual return whichever is earlier will be the last date for availing ITC in relation to the said invoices issued by the corresponding suppliers during the period from July, 2017 to March, 2018.