Random Light Schedule - how does it work?

How does the random function at schedule work?
I can select a time to turn on my lights, e.g. 20:00 o‘clock.
When I select 30 min. for random time what does it mean?
Will the random time start at 20:00, so that at latest at 20:30 the lights will turn on?
How does it work for turn-off?
Is the time I enter the starting point for random turn-off or the latest?

Besides I would like to understand the interaction between time selected and random time.
I experienced, in some cases that the light turns on but not off anymore. Could it be that the turn-off random time I choose is somehow overlapping with the turn-on random time?
Please explain the function or link to a explanation would be very helpful.