Here's why I'd want the 400 K's 27 hour limit to expire at sunset.
The 400 K is usually done with the idea of riding straight through without a formal sleep period. This means going 24+ hours and that means riding through the entire night.
My preference is to be as wide awake as possible, when riding at night. A lot more can go wrong at night and one should have their wits about them. Thus I'd opt for arranging the dark hours towards the beginning of the ride.
The sunrise will give the body a boost to make up for any time lost by being extra cautious at night.
The real downer comes at a second sunset. That's why I prefer to end any misery before then.