I don't understand how this works on a 20 hour cycle.

  Normal day: Spawn 10 hours after sunrise
  Early rising sun: Spawn 10 hours after sunrise
  Constant sun: Spawn every 20 hours

Does it reset back to zero after a cue of darkness? That's the only thing I can think of.

> A circadian clock must be self-sustained and internally driven, as the 20-hour cycle of the jellies’ spawning is. It must also be regulated by an environmental stimulus such as light; while the jellies’ spawning clock can run on a 20-hour cycle under persistent light in the lab, in nature it resets every day.

Sounds like that is what happens.