code: func (elt TimeMeddler) PreWrite(field interface{}) (saveValue interface{}, err error) {
  	switch tgt := field.(type) {
  	case time.Time:
  		if elt.ZeroIsNull && tgt.IsZero() {
  			return nil, nil
  		}
  		return tgt.UTC(), nil
  
  	case *time.Time:
  		if tgt == nil || elt.ZeroIsNull && tgt.IsZero() {
  			return nil, nil
  		}
  		return tgt.UTC(), nil
  
  	default:
  		return nil, fmt.Errorf("meddler.TimeMeddler.PreWrite: unknown struct field type: %T", field)
  	}
  }
docs: // PreWrite is called before an Insert or Update operation for fields that have a TimeMeddler