 If you liked
Suzumiya Haruhi no Shoushitsu
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 ...then you might like
Boku dake ga Inai Machi
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Both anime involves timetravelling and correction of either past mistakes or realities.
In both a male character tries to figure out how everything shoild be instead of what there is. In both cases the key is a girl.... Anyway both really worth watching due to its its tricky plot and touching story.