Can I issue a single invoice for retainer + mid-cycle add-on?

Yes, but only if you document service dates clearly for each line item. Split invoices are simpler and reduce audit risk. If you must combine, include service-date ranges and tax calculations for each line separately, so auditors can verify tax was applied correctly to each service window.

Does GST in Singapore apply to the invoice date or supply date?

GST tax point is the earlier of invoice date or supply date. A retainer supplied 1–30 June has a tax point of 1 June, even if invoiced later. A mid-cycle add-on supplied 15–30 June has a tax point of 15 June. If both are on one invoice dated 1 June, GST technically applies at 1 June for both—but this risks double-taxation if the retainer was already GST-invoiced separately. Two invoices avoid this.

What if the client's jurisdiction changes mid-month?

Split the invoice into two blocks: one for the old jurisdiction (with its tax rate), one for the new. Calculate tax separately for each block based on the service dates in that jurisdiction. Most platforms don't handle this, so you'll likely need to override or issue two separate invoices.

How do I record this in GL so my accountant doesn't flag it?

Create separate GL entries for each service date block and corresponding tax line, even on a single invoice. This proves you tracked service dates separately. When you file your GST/SST return, you can then reconcile tax by service-date period, not just by invoice.

Recurring billing tax GST SST proration