final SmileDeltaTermStructureDataBundle data, final int daysForward) {
ArgumentChecker.isTrue(daysForward == 1 || daysForward == -1, "daysForward must be either 1 or -1"); // TODO: Update signature of function to take ForwardOrBack
final InstrumentDerivative instrumentToday = definition.toDerivative(date, yieldCurveNames);
final ZonedDateTime horizonDate = date.plusDays(daysForward);
final double shiftTime = TimeCalculator.getTimeBetween(date, horizonDate);
final TodayPaymentCalculator paymentCalculator = TodayPaymentCalculator.getInstance(shiftTime);
final InstrumentDerivative instrumentTomorrow = definition.toDerivative(horizonDate, yieldCurveNames);
final MultipleCurrencyAmount paymentToday = instrumentToday.accept(paymentCalculator);
final SmileDeltaTermStructureDataBundle tomorrowData = FX_OPTION_ROLLDOWN.rollDown(data, shiftTime);
final PresentValueBlackSmileForexCalculator pvCalculator = PresentValueBlackSmileForexCalculator.getInstance();
return subtract(instrumentTomorrow.accept(pvCalculator, tomorrowData), instrumentToday.accept(pvCalculator, data)).plus(paymentToday);