What are the transitional rules for invoices or receipts issued  before January 1, 2018?

If the supply of goods or services takes place on or after January 1, 2018, any invoices issued before January 1, 2018 will be ignored for VAT purposes, and the supply will be subject to VAT. 

If an invoice is issued before January 1, 2018 without including VAT for a supply that is to be performed after January 1, 2018, the supplier must issue an additional invoice specifying the VAT amount.