But time management is more about knowing how long you need to do activities and planing accordingly. You practice by doing an estimation, do the thing, see how long you took and next time correcting accordingly. And there is no shame in using alarm clocks to remind you of the important bits.

That said, mental conditions like ADHD can make this incredibly hard. But in general you don’t keep the clock in your head, you keep stuff in your head and look at the clock every now and then.