[tmoertel]

> It's quite hard to, for example, change the system time when running a test which often means that you have to abstract out that part from the method you're testing and pass it through as a parameter.

Passing the time into your function isn't necessarily a thankless chore, however. It's actually quite similar to strengthening your induction hypothesis when doing a proof. Now your function doesn't just claim to work correctly for one time (the implicit clock time) but for all times. This stronger claim (if true) makes it easier to reason about the code that relies on the function, including not only the testing code but also the rest of your application code.