I just noticed this issue. I've never reported a bug before, so I don't know if this is a known problem, but it's reproducible for me, at least.
When I go to my profile, then go to the Reputation graph page, clicking on a date will show me the reputation changes for the next day. Or at least, they're labeled as the next day. (Not sure if the problem is in the data or the labeling).
Is anyone else seeing this bug?