code: func (pc *Periodic) cmpHourOfDay(ref time.Time) time.Duration {
  	pStart := pc.shiftTimeByDays(ref, 0)
  	return ref.Sub(pStart)
  }
docs: // cmpHourOfDay compares ref to Periodic occurring in the same day as ref.
  // The return value is less than, equal to, or greater than zero if ref occurs
  // before, equal to, or after the start of Periodic in the same day.