SpoilerWell, this is all up to Schazer, but lvl/x is easier to keep track of, so most likely that one.
And as for what defines a day, figure a lot of your operations are going to take place at night, so resetting at midnight doesn't make any sense. Every 24 hours is ill-defined, though, so you can probably expect your skills to reset when you've had a realistic amount of sleep/rest, regardless of time of day.