Unless you are working directly with something like reading the signals off the clock on the motherboard you are bound to deal with third-party code when dealing with clocks.
What's the point of your home-made timer if it doesn't faithfully emulate the actual clock that's going to be used in your actual program? How will you have confidence that your tests are even doing anything useful?
There are plenty of things that might be out of your control. If you try to emulate time. For example, the system, when using a realistic timer may decide to do some context switches, seeing how your code is blocking waiting on some wakeup event, while if you test against a timer that tries to speed things up those context switches won't happen. And there could be plenty examples of similar things.
If not, then how are you going to implement your home-made timer? Is your home-made timer going to be an actual timer (i.e. it will have to read time, thus relying on third-party code to do it) or will it pretend that time doesn't exist (and thus test something different from what your code is actually supposed to do)?
Your ideas about "substituting" system clock with hand-written code are out of reach for most developers out there. Even those who theoretically could embark on such a task probably won't because that's just too much effort.
What I'm getting at: the whole idea of mocking time for testing is a fool's errand. You'll be testing things that you don't actually need and won't be testing that actually need testing.
What's the point of your home-made timer if it doesn't faithfully emulate the actual clock that's going to be used in your actual program? How will you have confidence that your tests are even doing anything useful?
There are plenty of things that might be out of your control. If you try to emulate time. For example, the system, when using a realistic timer may decide to do some context switches, seeing how your code is blocking waiting on some wakeup event, while if you test against a timer that tries to speed things up those context switches won't happen. And there could be plenty examples of similar things.
If not, then how are you going to implement your home-made timer? Is your home-made timer going to be an actual timer (i.e. it will have to read time, thus relying on third-party code to do it) or will it pretend that time doesn't exist (and thus test something different from what your code is actually supposed to do)?
Your ideas about "substituting" system clock with hand-written code are out of reach for most developers out there. Even those who theoretically could embark on such a task probably won't because that's just too much effort.
What I'm getting at: the whole idea of mocking time for testing is a fool's errand. You'll be testing things that you don't actually need and won't be testing that actually need testing.