Starlight Glimmer went to the past to stop Rainbow Dash from performing her sonic rainboom, causing Equestria's future to change. Twilight managed to stop her by teaching her the true meaning of friendship. To be honest, the best timelines in that episode was King Sombra's crystal war, followed by Queen Chrysalis conquering Equestria. I made a Time Eater and In Our Town mashup based on Starlight and Twilight's time travel.