Export of goods & Milestone Invoicing

In case of  goods exporting from UAE to a foreign customer, Invoice are issued on a milestone basis based on progress of manufacturing of material but physical goods(final product) is moved out of country upon completion of manufacturing (Which in some cases requirer 6 months or a year). Zero rate supplies to be declared after actual export(goods moved out of country) or at the time of issuing milestone Invoices?? In case of time of actual export, how should milestone Invoices be treated (Zero rated/Taxable)?