I'm not understanding the recent activity date ranges at all. For example, today around 12PM I had 105 rep for the day. I was hoping to reach the cap when I got home from work :D but now I click "today" and I've got 25. I don't get it. When I click "Yesterday" it shows me posts from 2 hours ago. That's not yesterday, so that to me is a bug. Unless days are shorter in SO world of course. That's fine, but perhaps they should not be called "today" and "yesterday" if they don't start and end say, oh, around 24 hours apart. If I answered something today, I feel it should be in today, not moved out to yesterday after a few hours.
Days are defined in UTC.
The day ends at Midnight UTC, more commonly known as:
Notice these are Standard Times not, Daylight Savings Times. Therefore if you are in the Pacific Standard Timezone, and you post something at 2PM, at 5PM, it will be considered yesterday.