Yes, this is totally correct. I regularly have AONE4s issued, starting in CPT, and book the first two sectors (CPT-JNB-LHR) on the date I require, then put in dummy dates for the rest of the tickets, subsequently changing them when I know my actual travel itinerary. As long as the airport codes remain unaltered, you can change the dates without an additional fee (but only after making your first journey). You are only limited by each airlines' timetables: AA & JL 331 days ahead, BA & CX 355 days ahead. So effectively you have another 11+ months travel after your first international sector.