It uses a hardware clock, one that pauses during sleep. There is no tick.

If you wanted to see how time impacts the program, you'd prob change fns like calculate_tcp_clock to take uptime as an argument so that you could sanity check it.

Yes. I do mean designing software to make it testable.

The code that uses that value can be run in an environment where that value can be controlled.

I have written code that does this same thing and built a test harness for it.

We're talking about a company that produces the hardware their OS is running on. I'm sure they can find a way to make the hardware clock run faster.