something that bugs me about the Equestria Girl's mirror portal

The Equestria Girls movie's while not that great in my opinion did introduce one of my favorite characters, Sunset Shimmer. but with her introduction comes a disparity in the line of events. how can a Sunset who's high-school age be a student of Celestia before Twilight and then have Twilight grow up all while Sunset stays in high-school? I've seen this line of events tackled many different ways all depending on the narrative, most I find more confusing than necessary. rarely do I see authors attempt the idea of different time rates. maybe because it's perceived as too sci-fi. but we know for a fact time is not constant. so can this idea be implemented in writings? easily.

first if this concept is to be used we'll need to figure out at what differing rate each side is moving at. human side is easy we know Sunset is in high school and that she's there for 30 moons, or roughly 2.5 years. cause that's how long she thought she had to be there.
next we'll need a corresponding year length for the equestrian side. first it'll move faster since we need to get from; Sunset leaving, Twilight becoming a student, to Twilight becoming an alicorn and going to the princess summit. first I'll assume Twilight is an adult but also still young enough to be confused for a high-schooler so 18-19, I'll stick to 18. next subtract the age assuming she started school so I'll guess 5 and that puts us at 13 years. now remember the 30 moons is a definite equestrian side thing so we need to add years so it's a number evenly divisible by 2.5, so add in 2 years between Sunset leaving and Twilight becoming a student. we can say then 15 years total have past in Equestria, from the time Sunset left to when she returns. so from this we get 30 moons on earth equal to 180 moons on Equestria. with these rates we simplify it to the portal opening every 6 months on earth to every 30 Equestrian months.
poor sunset with this set up she could have gone home so much sooner if she only knew.
this can go even further explaining why the portal only opens every so often, cause remember time is a non-constant it bends and flexes normally. so it's an easy leap of logic to conclude the portal opens when the time rates begin to sync up. then stays open until there's too much disparity between the two time streams. there's more math that can be done but you'll need to use sine curves and calculus, which I don't feel like doing.

tl;dr?
assuming time moves differently between sides of the mirror Sunset could have hung out in high-school for two and a half years while Equestria whizzed by long enough for Twilight to grow up and become an alicorn.

What I see happen most though is either it's never brought up or something along the lines of "Sunset mysteriously gets flung to the past and strangely becomes even younger and can't reenter the portal in the normal 30 moons" all so Twilight is old enough for the events of Equestria Girls. I feel relative time works so much better to explain the line of events. So simple an author can just mention it in passing, and never have to break a reader's suspension of disbelief.