I remember looking for that before and never did find anything. 
Just my thinking on it was the retention is the minimum time retained. So if a change happened at 11:50 pm, it would be way too problematic if it was deleted at 12:01 am. I would guess 1 day would be the same as 24 hours since it needs to be retained at least one day. And even 11:50 am would be half a day, not a day. But I never did find anything official so that's what I made up 🙂
I did notice that things can end up in there longer than the retention period - not real bad but things aren't deleted right when meeting the retention period even though that cleanup process runs every 30 mins. Saw that when I was trying to figure out the same thing.

Sue