The Central Board of Excise and Customs (CBEC) on Saturday said the last date for GSTR-3B return filing for December has been extended by two days to January 22.CBEC said in a tweet on Saturday that the deadline to file the return for December in Form GSTR-3B was January 20 which has been extended up to Monday. GSTR-3B is a simple return form introduced by the CBEC after the roll out of the goods and services tax (GST) from July 1. It is meant to capture details of outward and inward supplies. The CBEC has already notified the date for filing of return for the months of January, February and March.It is mandatory for all entities registered for the GST to file Form GSTR-3B.