Time of supply determines when GST liability arises, defining invoice, payment, voucher, reverse charge and rate change rules. The time of supply fixes when GST liability arises: for goods, it is the earlier of invoice issue (or last prescribed date for issue) and receipt of payment, subject to a notification relaxing advance-payment timing; for services, it is the earlier of invoice issue (if within prescribed period) or receipt of payment, otherwise the earlier of provision of service or receipt of payment, or the date the recipient records the service. Residual rules treat the periodical-return due date or actual tax payment as time of supply; reverse-charge and rate-change sequencing and invoice-issue timelines are also prescribed.