I was wondering this too actually.
Let me test it for you
Every "10" on the slider is about 25 seconds, which means a day is 4.5-5 hours.
The second hour starts about 1750
the third starts about 3500
the forth is about 5200
If you click the scroll bar on the weather maker, it will move it ahead or back by one in game hour, which is about 699 units.
I am guessing 0 is noon, therefore the halfway point at about 3499 will be midnight.
I am unsure of which in game time the sun sets or rises at though.