code: func (m TimeOfDayMapper) ID(ti ...interface{}) (rowIDs []int64, err error) {
  	t := ti[0].(time.Time)
  	daySeconds := int64(t.Second() + t.Minute()*60 + t.Hour()*3600)
  	return []int64{int64(float64(daySeconds*m.Res) / 86400)}, nil // TODO eliminate extraneous casts
  }
docs: // ID maps a timestamp to a time of day bucket