code: func (ns *Namespace) Duration(unit interface{}, number interface{}) (_time.Duration, error) {
  	unitStr, err := cast.ToStringE(unit)
  	if err != nil {
  		return 0, err
  	}
  	unitDuration, found := durationUnits[unitStr]
  	if !found {
  		return 0, fmt.Errorf("%q is not a valid duration unit", unit)
  	}
  	n, err := cast.ToInt64E(number)
  	if err != nil {
  		return 0, err
  	}
  	return _time.Duration(n) * unitDuration, nil
  }
docs: // Duration converts the given number to a time.Duration.
  // Unit is one of nanosecond/ns, microsecond/us/µs, millisecond/ms, second/s, minute/m or hour/h.