Hi there! ^.^ I'm not 100% sure about the situation because I don't usually tt backwards. But from my experience, it may change the date that she's leaving. I really am not completely sure about that one. ;-; So if u want to be safe I would suggest just tt'ing forward just incase because I'm not sure what might happen. Also, it is recommended to just go with the in-game clock because it's easier and u won't risk messing up any dates or anything in any of your other games if u use that one. I hope that made sense and was of help to u. c:
