I have the Daylight Length set to 16 Hours as I like a longer night. That used to match with the trader opening time (at 0600), but now traders open at 0400.
I was playing a Day 7 Horde. I didn't make a horde base, so I was fighting while mobile, basically doing laps around Trader Rekt's place the entire time.
At 0400 -- during the horde night -- Trader Rekt opened up. I opened his gates ... he might as well join the fun. I could run around inside the compound during the horde night. Sometimes I would get teleported out and sometimes I wouldn't.
I want to think there's a possible exploit here in that I might be able to spend part of a horde night in an indestructible fortress. I'm tempted to set the Daylight Length to 12 hours for the next horde night.
I suspect what would be best for the game is if the Traders open at whenever the day starts, rather than a fixed time.