code: public static <T extends java.util.Date> T setDays(final T date, final int amount) {
          return set(date, Calendar.DAY_OF_MONTH, amount);
      }
docs: Copied from Apache Commons Lang under Apache License v2.
  <br />
  
  Sets the day of month field to a date returning a new object.
  The original {@code Date} is unchanged.
  
  @param date  the date, not null
  @param amount the amount to set
  @return a new {@code Date} set with the specified value
  @throws IllegalArgumentException if the date is null
  @since 2.4