code: public LocalDateTime plusDays(long days) {
          LocalDate newDate = date.plusDays(days);
          return with(newDate, time);
      }
docs: Returns a copy of this {@code LocalDateTime} with the specified number of days added.
  <p>
  This method adds the specified amount to the days field incrementing the
  month and year fields as necessary to ensure the result remains valid.
  The result is only invalid if the maximum/minimum year is exceeded.
  <p>
  For example, 2008-12-31 plus one day would result in 2009-01-01.
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param days  the days to add, may be negative
  @return a {@code LocalDateTime} based on this date-time with the days added, not null
  @throws DateTimeException if the result exceeds the supported date range