1.
Your service is continuous supply of service. Pl refer Sec 2(33) of CGST Act 2017 as under:
Sec 2 (33) “continuous supply of services” means a supply of services which is provided, or agreed to be provided, continuously or on recurrent basis, under a contract, for a period exceeding three months with periodic payment obligations and includes supply of such services as the Government may, subject to such conditions, as it may, by notification, specify;
The determination of stage of completion of services is an abstract one, unless specifically defined by contract, unlike in the case of goods where the volume of goods supplied can be easily tracked/ identified. Hence, a contract for supply of service spanning over a definite period has been treated as a continuous supply, so that the tax dues are collected periodically.
The law categorically provides for time limit to issue invoices as under:
(a) where due date of payment is ascertainable: On or before the due date of payment;
(b) where the due date of payment is not ascertainable: Before or at the time of receipt of payment;
(c) where the payment is linked to the completion of an event (milestones): On or before the date of completion of that event.