Swift repeat rule date calculator

RepeatRuleCalculator.swift

// based on: http://stackoverflow.com/a/31598663/234609
// Original gist: https://gist.github.com/jim-rhoades/ea0c5b241ec77a37f86b
// (paste this into an Xcode playground for testing)

import UIKit

let currentCalendar = Calendar.current
var dateComponents = DateComponents()
dateComponents.day = 31
dateComponents.month = 1
dateComponents.year = 2015
guard let pastDate = currentCalendar.date(from: dateComponents) else {
    fatalError("failed to create NSDate")
}

var optionalNow: Date? = Date()

// assume "now" is March 15, 2015
// or comment this out to test using the current date/time
let marchDateComponents = DateComponents()
dateComponents.day = 15
dateComponents.month = 3
dateComponents.year = 2015
optionalNow = currentCalendar.date(from: dateComponents)


guard let now = optionalNow else {
    fatalError("optionalNow was nil")
}

let repeatUnit = Calendar.Component.month
let repeatInterval = 3

var deltaComponents = currentCalendar.dateComponents([repeatUnit], from: pastDate, to: now)
let deltaValue = deltaComponents.value(for: repeatUnit)!


// MODIFIED VERSION
let repeatsToNowPlusOne = deltaValue - (deltaValue % repeatInterval) + repeatInterval

var nextDateComponents = DateComponents()
nextDateComponents.setValue(repeatsToNowPlusOne, for: repeatUnit)

let nextDate = currentCalendar.date(byAdding: nextDateComponents, to: pastDate)