import UIKit

// Requires iOS 11+ / macOS 10.13+
extension ISO8601DateFormatter {
    static var ISO8601WithFractionalSeconds: ISO8601DateFormatter {
        let formatter = ISO8601DateFormatter()
        formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
        return formatter
    }

    static var ISO8601NoFractionalSeconds: ISO8601DateFormatter {
        let formatter = ISO8601DateFormatter()
        formatter.formatOptions = [.withInternetDateTime]
        return formatter
    }
}

/*
 * You can create a single parser function to handle datetimes with and without fractional seconds
 */
func getDateFrom(from dateString: String) -> Date? {
    if let date = ISO8601DateFormatter.ISO8601WithFractionalSeconds.date(from: dateString) {
        return date
    }
    if let date = ISO8601DateFormatter.ISO8601NoFractionalSeconds.date(from: dateString) {
        return date
    }
    return nil
}

func timeit(task: () -> Date?) -> Date? {
    let start = DispatchTime.now()
    let date = task()
    let end = DispatchTime.now()
    let dt = Double(end.uptimeNanoseconds - start.uptimeNanoseconds) / 1_000_000
    print("Runtime: \(dt) ms")
    return date
}

let datetimeWithoutFractional = "2019-09-30T12:34:56Z"
let datetimeWithFractional = "2019-09-30T12:34:56.789Z"

print("Parsing fractional seconds...")
let parsedWithFractional = timeit { getDateFrom(from: datetimeWithFractional) }
print("Result: \(parsedWithFractional)")

print("Parsing no fractional seconds...")
let parsedWithoutFractional = timeit { getDateFrom(from: datetimeWithoutFractional) }
print("Result: \(parsedWithoutFractional)")

/*
 * Results of a few runs... from a 2017 MacBook Pro (13 inch)
 *
 * Parsing fractional seconds...
 * Runtime: 36.45441 ms
 * Result: Optional(2019-09-30 12:34:56 +0000)
 * Parsing no fractional seconds...
 * Runtime: 1.881623 ms
 * Result: Optional(2019-09-30 12:34:56 +0000)
 *
 * Parsing fractional seconds...
 * Runtime: 38.708586 ms
 * Result: Optional(2019-09-30 12:34:56 +0000)
 * Parsing no fractional seconds...
 * Runtime: 1.904698 ms
 * Result: Optional(2019-09-30 12:34:56 +0000)
 *
 * Parsing fractional seconds...
 * Runtime: 36.550756 ms
 * Result: Optional(2019-09-30 12:34:56 +0000)
 * Parsing no fractional seconds...
 * Runtime: 2.093251 ms
 * Result: Optional(2019-09-30 12:34:56 +0000)
 *
 * Parsing fractional seconds...
 * Runtime: 35.279147 ms
 * Result: Optional(2019-09-30 12:34:56 +0000)
 * Parsing no fractional seconds...
 * Runtime: 1.863788 ms
 * Result: Optional(2019-09-30 12:34:56 +0000)
 *
 * Parsing fractional seconds...
 * Runtime: 37.178746 ms
 * Result: Optional(2019-09-30 12:34:56 +0000)
 * Parsing no fractional seconds...
 * Runtime: 1.925432 ms
 * Result: Optional(2019-09-30 12:34:56 +0000)
 *
 * Average results:
 * With fractional: ~37 ms
 * Without fractional: ~2 ms
 */