What is the time period within which invoice has to be issued in a case involving continuous supply of services under GST?

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In case of continuous supply of services,
(a) where the due date of payment is ascertainable from the contract, the invoice shall be issued before or after the payment is liable to be made by the recipient but within a period prescribed in this behalf whether or not any payment has been received by the supplier of the service;
(b) where the due date of payment is not ascertainable from the contract, the invoice shall be issued before or after each such time when the supplier of service receives the payment but within a period prescribed in this behalf;
(c) where the payment is linked to the completion of an event, the invoice shall be issued before or after the time of completion of that event but within a period prescribed in this behalf.