when is PST and GST taxes are calculated?

during Purchase order entry - tax expln code & area is entered,buyer should be diligent to enter the tax area & tax expln code during PO entry, however the final taxes are calculated during voucher match. based on the tax expn code & area specified on the invoice which is corrected if incorrectly mentioned on the PO. we have voucher logging turned on for all invoices, based on the DMAAIs the respective a/cs are impacted. GST and PST are calculated based on P4008 setup and tax expln codes and are codes and the accounts to hit are based on DMAAIs