acknowledge order report amazon mws

I am using "GetReportList" api with report list type as "_GET_ORDERS_DATA" to pull order reports from amazon. But I want to pull only new orders. How can I use the "acknowledged" field to make sure that I pull only new orders(which were not previously pulled).I observed that the "acknowledged" field is true by default. Please let me know if there is a way to pull new orders only(I am trying to avoid using timestamp here)

Thanks