Hi Diane,

Tax is calculated for invoices and credit notes based on existing tax tables. If you change your tax tables to the new GST rate, then indeed tax will always calculate based on the new rate, regardless of the invoice date.

If you were desperate, you could create a new tax table and keep the old 7% rate, and use this table specifically for back-dated invoices. But this might break up your reporting (say the tax tracking reports) and might not be a good solution for you.