I can see your point here, but since time is fundamentally an external datum provided by the system, in my view there's no reason to screw with Haskell's intern

I can see your point here, but since time is fundamentally an external datum provided by the system, in my view there's no reason to screw with Haskell's internals when you can just produce the necessary simulated environment in which your runtime lives. Moreover, by doing it this way you can use exactly the same code in testing as in production, reducing the risk of something going wrong when you change from the former to the latter codebase.