code: public long addWrapField(long instant, int amount) {
          return set(instant, FieldUtils.getWrappedValue(get(instant), amount, 0, iDivisor - 1));
      }
docs: Add the specified amount to the specified time instant, wrapping around
  within the remainder range if necessary. The amount added may be
  negative.
  
  @param instant  the time instant in millis to update.
  @param amount  the amount to add (can be negative).
  @return the updated time instant.