Folklore is generally full of plot-holes, so I have set up my own rules for what exactly happens when entering elven territory within this universe.
Basically, elves live in a sort of parallel dimension, and going from their world to the human world can distort time, so you’ll arrive much later than when you started. Travelling with an elf will usually counter most of the distortions. But as they are very long lived, their idea of acceptable time loss can differ quite much from what people in our society find acceptable.
Travel without elven help and you’re on your own. Not a particularly good idea (other bad ideas include eating food or drinking anything while in their dimension).

Being a troll, Mirabell also is long-lived and much better to naturally counter the distortions and arrive close to the right time. So she’s not particularly worried to enter elven land (as long as there’s no conflicts between her kind and the elves).