Day ends at midnight (I know, duh!)
I know the day really does end at midnight, but I often use Duolingo between 11 PM and 1 AM. I wish that session would count towards one day. When the coins split up between days, I get a little sad. Is there a way to change this? I'm sorry if this is too silly a request!
In psychology speak: if it bothers you it should be addressed. When I work around midnight, I see the same thing. The positive side of this is that you start your day (even if it is the night) with a certain amount of coins. At the end you will level up as fast as you would have done if the coins had been allocated on the day you started to work.