Input tax credit can be availed immediately on the receipt of goods and service and proper Invoice.
However, as per the provision of Section 16(4) Input tax cannot be availed
a) Where invoice date is old more than one year or
b) After filing of return for the month of September following the end of financial year to which such invoice or invoice relating to debit note pertains
c) Furnishing the annual return whichever is earliest.
It means ITC would not be available on invoice issued by supplier or service provider between 01.09.2017 to 31.03.2018 after 20.10.2018 or date of annual filing of Return (i.e on or before 31.12.2018), whichever is earlier.