import Foundation

func formatter(with dateFormat: String) -> DateFormatter {
    let formatter = DateFormatter()
    formatter.locale = Locale(identifier: "en_US_POSIX")
    formatter.dateFormat = dateFormat
    formatter.timeZone = TimeZone(secondsFromGMT: 0)
    return formatter
}

let iso8601Formatter = formatter(with: "yyyy'-'MM'-'dd'T'HH':'mm':'ss'Z")
let iso8601WithFractionalSecondFormatter = formatter(with: "yyyy'-'MM'-'dd'T'HH':'mm':'ss.SSSS'Z")

let gregorian = Calendar(identifier: .gregorian)
let utc = TimeZone(identifier: "UTC")!
let components = DateComponents(
    calendar: gregorian, timeZone: utc,
    year: 2019, month: 6, day: 30, hour: 3, minute: 31, second: 35
)

func check(from components: DateComponents, with nanosecond: Int) {
    print("--")
    print("Dircted nanosecond: \(nanosecond)")
    var components = components
    components.nanosecond = nanosecond
    let date = components.date!
    print("Actual nanosecond : \(gregorian.dateComponents(in: utc, from: date).nanosecond!)")
    print("Date: \(iso8601Formatter.string(from: date))")
    print("Date with fractional second: \(iso8601WithFractionalSecondFormatter.string(from: date))")
}

check(from: components, with: 999499976)
check(from: components, with: 999499977)