code: public long addWrapField(long instant, int amount) {
          int thisValue = get(instant);
          int wrappedValue = FieldUtils.getWrappedValue
              (thisValue, amount, getMinimumValue(), getMaximumValue());
          // copy code from set() to avoid repeat call to get()
          return instant + (wrappedValue - thisValue) * getUnitMillis();
      }
docs: Add to the component of the specified time instant, wrapping around
  within that component if necessary.
  
  @param instant  the milliseconds from 1970-01-01T00:00:00Z to add to
  @param amount  the amount of units to add (can be negative).
  @return the updated time instant.