the semantics you'd like the OS + standard library to provide would be some kind of gettime() call that returns a time thingie, and a secondsbetween(a,b) call t

the semantics you'd like the OS + standard library to provide would be some kind of gettime() call that returns a time thingie, and a secondsbetween(a,b) call that reliably tells you the time between the two time thingies. the fact that it doesn't already work this way is a design fail. all the nonsense about NTP and clock slew and monotonicity are implementation details that should be hidden below this layer.