Clockwork will try to override the settings about day length. It won't interfere with the 2 hour grace before night, unless you try to enable Sun Never Rises mode. Nor would it interfere with any perma-night modes, nor biter AI/settings.
So essentially, Clockwork's daylength settings may apply instead depending on the order in which the 2 mods try to modify it, or Pitch Black's might. Beyond that, I would not expect any interference.
