Every military plan has to be planned ahead. You can't tell what will happen in the meanwhile, so everything is planned around D-Day and H-Hour. So, everyone has accurate and flexible documentation. D-Day in Normandy turned out to be 6 June 1944. But D-Day was the date of every operation planned; for me, for instance, D-Day happened to be 9 September 1943 when I launched at Salerno.