The poster clearly states that "this is not a love story, it is a story about love." However, nowhere in the entire movie do we see, on-screen, anything that approximates love. Tom, the main character, falls for Summer, his boss's new secretary, and spends the rest of the movie trying to be with her. From what we see, Tom's affection for Summer can only be labeled as lust, because she is an emotionless flake devoid of any guiding rationality or common interests with Tom-- save, perhaps, in a singular obscure band. All this is made even more puzzling by the fact that, objectively, Summer (played by the curiously named Zooey Deschanel) is not very attractive. Despite his better judgment and the no doubt torrent of advice he likely received from his friends off-camera, Tom continues to pursue the relationship even though it can only lead to disaster.
Aiding Tom in his quest for low-hanging, rotten fruit is his prepubescent sister. The 12-year-old, bolstering the complete unbelievability of the movie, is an expert at love and relationships and despite the fact that she is probably still completing grammar workbooks in school, is able to offer perfectly-written sage-like advice whenever needed. If people in their 50s can't speak intelligently about love, I think it is fair to say that such a girl has never existed in human history, and therefore this "true-to-life" movie is fatally flawed by her inclusion. Further, since Tom works full time and likely graduated from college, he is probably more than 10 years his sister's senior, making for quite an odd and unexplained family dynamic.
Lastly, the movie ends with the Summer "day-clock" resetting to day 1 with the beginning of a new relationship, an outcome I forsaw but was hoping to avoid. Tom is still clearly emotional over Summer and instead of dealing with his feelings, he merely compartmentalizes them as he starts a new relationship. Therefore the moral of the story seems to be that feelings cannot be overcome, only ignored, and that the best solution to a bad break-up is a rebound.
Simply put, don't believe the hype: 500 days of summer is not about love or any other function that would normally happen on planet earth. It is 95 minutes of waiting because when the imaginative method of storytelling is removed, nothing of significance happens in the plot.