The deadline for claiming Input Tax Credit (ITC) for the financial year 2018-19 is the GSTR-3B filing for September 2019, which is due by 20th October 2019. This means taxpayers have until this date to reconcile their books with their filed returns and claim any eligible ITC that may have been missed or inadvertently excluded. It's essential to perform a GSTR-2A reconciliation to ensure all eligible credit is claimed before this final cut-off.