code: func (cmp *TimestampLessThan) MarshalJSON() ([]byte, error) {
  	return json.Marshal(&struct {
  		Variable          string
  		TimestampLessThan string
  	}{
  		Variable:          cmp.Variable,
  		TimestampLessThan: cmp.Value.Format(time.RFC3339),
  	})
  }
docs: // MarshalJSON for custom marshalling