I always used the invoice date unless the year-end is already sent to acct. It is quite common to have the GST report filed and then have an invoice come in that is dated within the GST report period. Just post as per usual using the invoice date as shown on invoice and next time you do the GST report it will be included. You should be using the balance sheet as the backup document to input your GST nbrs from.

ok so we didnt send the year end to accountants yet but we are starting to print out stuff for them... is it still fine?  the only thing i printed yet are my last GST stuff (income statement and Balance Sheet), last bank rec info and cheque log of last month and beginning of new year.  I dont think posting the invoice with the actual date will affect anything from there??

It is still ok. When sending to accountant you can make a note that G/L GST Paid on Purchases balance of XXX is due to invoices received after the remittance report sent in.