As per section 31 of the CGST Act, 2017 an invoice for supply of goods needs to be issued before or at the time of removal of goods for supply to the recipient where the supply involves movement of goods. However, in other cases, invoice needs to be issued before or at the time of delivery of goods or making available goods to the recipient.
Similarly, an invoice for supply of services needs to be issued before or after the provision of service but not later than thirty days from the date of provision of service.
Source: CBIC Website
