As the folks have said FDP and Duty Period are different: some examples.
You report at base to position by road to ABC to operate ABC - XYZ - ABC and then position by road back to base. Your FDP starts when you report for the surface movement to ABC. It ends when the aircraft comes to halt at ABC after the 2 flights. Your Duty Period ends when the taxi gets back to ABC at the end of the day.
You arrive in the office at 0800, your Duty Period starts. At 1200 Ops phone and ask you to operate Base - ABC - Base, report is 1300. Your FDP commences at 1300 and must end either at 0800 + the maximum permitted duty period, or 1300+ the maximum permitted FDP.
So if the max DP is 12 hours and the max Duty Period is 16 hours you have to stop at Midnight - Duty start + 16 - and you can't go in to discretion even though the FDP would end at 0100 - 1300 + 12 hrs.
Clear as mud?
Gets better. The use of the word "rostered" in the legislation and the Ops Manual should be regarded with some caution. Examples.
You are rostered to operate 2 sectors ABC - DEF - XYZ and nightstop. The FDP ends at Max FDP - 1 hr and the flt time XYZ - ABC is 2 hours.
Before you leave ABC you can't plan to go in to discretion to operate another sector XYZ - ABC at the end of the day, however, as soon as you move off blocks the company can ask you to operate that last extra sector and go in to discretion as the operation has commenced.
Your Ops Manual allows you to be rostered for a max of 4 sectors if you report between the hours of 0001 - 0600. You report at 0545 to operate 4 rostered short sectors with planned finish of 1200; FDP rostered is 6 hrs 15 mins, max FDP is 12 hours.
On your return to base the company ask you to operate another 2 sectors as a jet has gone tech downroute. Can you do it?
Some would say no, the max number of sectors is 4, however, that is at the planning i.e rostering stage, and once the operation commences that figure is no longer valid and you can operate the 6 sectors.
All down to interpretation of the legislation.