We have a designated TV time. If she's unwell or I've had a rough night, then designated TV time can extend to all day

The rule in our house is from 4pm DD1 can watch TV. She has known her numbers for a long time so we taught her to look at my digital bedside clock to see if its 4pm. So if she asked "can I watch tv?" the answer was "Is it four o'clock?". It then began to be a game for her to check the time (she'd often come out shaking her head, saying "Nooo it's only three o'clock") and more often than not now, if she wants TV, she knows to check first and she will generally turn on the radio instead.

Could that be something that might work for your DS? In the early days of us doing it, it was a real game for her so the distraction of running to look for the clock and check was enough to divert a tantrum over it.