I've mentioned before that time is hard. From leap days/seconds to time zone definition changes to GPS time resetting to 0 every Sunday, it's just not that simple.
But it's even worse than that. Consider the truly functional languages, such as Haskell, Erlang, and OCaml. Functions don't have side effects, because they can't. Except… One side effect you can't prevent, even in the most functional of languages, is the passage of time. The result of calling time.Now() before and after the function call will be different. And that's a side effect.
Of course, you can't change time. Unless you're running a simulation. Then things are different. If you're running a simulation then one of the first things you need to simulate is time. And then you need to rigidly control it. If you're running a distributed simulation you also need to make sure that all of the nodes in your system always agree on what time it is. And that's hard. Doable, but hard. If you don't, someone will figure out a way to deal with it or come up with a plausible (but wrong) explanation.
Back when I was doing aerospace work I was working with some folks doing simulations of the Northrop YF-23, and a big part of the testing was M vs.N air combat. This was a hard real time system, with most of the effort going into accurately simulating the performance and response of the YF-23s. Everything else in the simulation was just there to provide a realistic environment. And the pilots of other aircraft complained that at the beginning of the simulation their aircraft were "heavy" and unresponsive, but after a while they started to respond better. Turns out that wasn't the case. Instead, because it was a hard real-time system and everything had to happen within the fixed frame time, the scheduler, to make everything fit, would take the things at the end of the list of tasks and put some of them in the even frames and some in the odd frames, then tell them to simulate twice the time. While this is accurate, it's not quite right because it doubled the transport delay and latency, which the pilots saw as the aircraft being "heavy" and unresponsive. In reality it was that as the number of simulated objects shrank, eventually everything was run every frame, and the pilots felt better. Whether this was a bug or a feature is a different question, but this was only possible because the system had complete control over time in the simulation and could do whatever it needed to keep it in sync with wall time.
Another place that time as a side-effect can bite you is in unit tests. Luckily, unit tests are kind of like simulations, in that you can control time there as well. Or you can if you access time as an injected dependency. Then you can mock it and get time to do what you want, not what it ends up doing on its own. Which makes it possible to test all of those edge cases without writing long tests with random sleeps or ending up with flaky tests. And we all want to eliminate flaky tests.