code: func (s *RotationSchedule) CheckAndSetDefaults(clock clockwork.Clock) error {
  	if s.UpdateServers.IsZero() {
  		return trace.BadParameter("phase %q has no time switch scheduled", RotationPhaseUpdateServers)
  	}
  	if s.Standby.IsZero() {
  		return trace.BadParameter("phase %q has no time switch scheduled", RotationPhaseStandby)
  	}
  	if s.Standby.Before(s.UpdateServers) {
  		return trace.BadParameter("phase %q can not be scheduled before %q", RotationPhaseStandby, RotationPhaseUpdateServers)
  	}
  	if s.UpdateServers.Before(clock.Now()) {
  		return trace.BadParameter("phase %q can not be scheduled in the past", RotationPhaseUpdateServers)
  	}
  	if s.Standby.Before(clock.Now()) {
  		return trace.BadParameter("phase %q can not be scheduled in the past", RotationPhaseStandby)
  	}
  	return nil
  }
docs: // CheckAndSetDefaults checks and sets default values of the rotation schedule.