code: public long add(long instant, int amount) {
          instant = super.add(instant, amount);
          FieldUtils.verifyValueBounds(this, get(instant), iMin, iMax);
          return instant;
      }
docs: Add the specified amount of offset units to the specified time
  instant. The amount added may be negative.
  
  @param instant  the time instant in millis to update.
  @param amount  the amount of units to add (can be negative).
  @return the updated time instant.