message: changed subtract.cpp to use Interval::Invert
diff: @@ -65,18 +65,12 @@ interval_t SubtractOperator::Operation(interval_t left, interval_t right) {
  template <>
  date_t SubtractOperator::Operation(date_t left, interval_t right) {
  - right.months = -right.months;
  - right.days = -right.days;
  - right.micros = -right.micros;
  - return AddOperator::Operation<date_t, interval_t, date_t>(left, right);
  + return AddOperator::Operation<date_t, interval_t, date_t>(left, Interval::Invert(right));
  }
  template <>
  timestamp_t SubtractOperator::Operation(timestamp_t left, interval_t right) {
  - right.months = -right.months;
  - right.days = -right.days;
  - right.micros = -right.micros;
  - return AddOperator::Operation<timestamp_t, interval_t, timestamp_t>(left, right);
  + return AddOperator::Operation<timestamp_t, interval_t, timestamp_t>(left, Interval::Invert(right));
  }
  template <>
  