Mental time travel (MTT) is the ability to mentally project oneself backward or forward in time in order to remember an event from one's personal past or to imagine a possible event in one's personal future. Past and future MTT share many similarities, and there is evidence to suggest that the two temporal directions rely on a shared neural network and similar cognitive structures. At the same time, one major difference between past and future MTT is that future as compared to past events generally are more emotionally positive and idyllic, suggesting that the two types of event representations may also serve different functions for emotion, self, and behavioral regulation. Here, we asked 158 participants to remember one positive and one negative event from their personal past as well as to imagine one positive and one negative event from their potential personal future and to rate the events on phenomenological characteristics. We replicated previous work regarding similarities between past and future MTT. We also found that positive events were more phenomenologically vivid than negative events. However, across most variables, we consistently found an increased effect of emotional valence for future as compared to past MTT, showing that the differences between positive and negative events were larger for future than for past events. Our findings support the idea that future MTT is biased by uncorrected positive illusions, whereas past MTT is constrained by the reality of things that have actually happened.