code: public Date changeDate(Date date, int delta) {
          return adjustDate(date, Calendar.DATE, delta);
      }
docs: Roll a date forward or back a given number of days. Only the days are
  modified in the associated date. If the delta is positive, then the date
  is rolled forward the given number of days. If the delta is negative,
  then the date is rolled backward the given number of days.
  
  @param date The initial date from which to start.
  @param delta The positive or negative integer value of days to move
  
  @return The new {@link Date} instance reflecting the specified change