@Sereg I just mean that the very inclusion of repeatable time travel sets off a million different types of Fridge Logic unless you have a very detailed explanation of all of the rules and limitations. For example, you said that they can't change the past, but that could be because You Already Changed The Past, or because they can only see into the past, or because they become unable to physically interact with anything in the past, or because time is self-correcting, etc. It is not something to be brought up casually, and most of those scenarios have tons of loopholes that need to be accounted for.
I'm fully aware. There's a reason that I usually avoid writing Time Travel but when I actually need to anyway, I'm going to have some fun with it. The spell Twilight used results in
You Already Changed The Past and anypony affected by it becomes immune to future castings. The spell the Discord used is repeatable, but you are unable to interact with anything. Travel to the future allows you to interact with stuff, but it's only a
possible future. When the timeline stabalises, only some copies of younger travelers will remain. That's why Doctor Whooves's younger self (of course, he wasn't named that yet) was in the battle for Appleloosa and Derpy's younger self witnessed the events of Cutie Pox and a later version of herself participated in the Best Young Fliers competition.