rfc3339.swift

//
//  rfc3339.swift
//  Alwyzon for iPhone and iPad
//

import Foundation

/// Parse RFC 3339 date string to NSDate
///
/// :param: rfc3339DateTimeString string with format "yyyy-MM-ddTHH:mm:ss.SSS"
/// :returns: NSDate, or nil if string cannot be parsed
public func dateForRFC3339DateTimeString(rfc3339DateTimeString: String) -> NSDate? {
    let formatter = getThreadLocalRFC3339DateFormatter()
    return formatter.dateFromString(rfc3339DateTimeString)
}

/// Generate RFC 3339 date string for an NSDate
///
/// :param: date NSDate
/// :returns: String
public func rfc3339DateTimeStringForDate(date: NSDate) -> String {
    let formatter = getThreadLocalRFC3339DateFormatter()
    return formatter.stringFromDate(date)
}

// Date formatters are not thread-safe, so use a thread-local instance
private func getThreadLocalRFC3339DateFormatter() -> NSDateFormatter {
    return cachedThreadLocalObjectWithKey("com.alwyzon.getThreadLocalRFC3339DateFormatter") {
        let en_US_POSIX = NSLocale(localeIdentifier: "en_US_POSIX")
        let rfc3339DateFormatter = NSDateFormatter()
        rfc3339DateFormatter.locale = en_US_POSIX
        rfc3339DateFormatter.dateFormat = "yyyy'-'MM'-'dd'T'HH':'mm':'ss.SSS"
        rfc3339DateFormatter.timeZone = NSTimeZone(forSecondsFromGMT: 0)
        return rfc3339DateFormatter
    }
}

/// Return a thread-local object, creating it if it has not already been created
///
/// :param: create closure that will be invoked to create the object
/// :returns: object of type T
private func cachedThreadLocalObjectWithKey<T: AnyObject>(key: String, create: () -> T) -> T {
    let threadDictionary = NSThread.currentThread().threadDictionary
    if let cachedObject = threadDictionary[key] as? T {
        return cachedObject
    }
    else {
        let newObject = create()
        threadDictionary[key] = newObject
        return newObject
    }
}

rfc3339Tests.swift

//
//  rfc3339Tests.swift
//  Alwyzon for iPhone and iPad
//

import UIKit
import XCTest
@testable import Alwyzon

class RFC3339Test: XCTestCase {
    
    let en_US_POSIX = NSLocale(localeIdentifier: "en_US_POSIX")
    let utcTimeZone = NSTimeZone(forSecondsFromGMT: 0)
    
    lazy var calendar: NSCalendar = {
        let cal = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
        cal.locale = self.en_US_POSIX
        cal.timeZone = self.utcTimeZone
        return cal
    }()
    
    let calendarUnits: NSCalendarUnit = [.Year, .Month, .Day, .Hour, .Minute, .Second]
    
    func testUTCStringToDate() {
        let string = "1967-01-17T16:34:25.123"
        let optDate = dateForRFC3339DateTimeString(string)
        XCTAssertNotNil(optDate)
        if let date = optDate {
            let components = calendar.components(calendarUnits, fromDate: date)
            XCTAssertEqual(1967, components.year)
            XCTAssertEqual(1, components.month)
            XCTAssertEqual(17, components.day)
            XCTAssertEqual(16, components.hour)
            XCTAssertEqual(34, components.minute)
            XCTAssertEqual(25, components.second)
        }
    }
}

See also:

• https://developer.apple.com/documentation/foundation/date/3766420-iso8601format
• https://sarunw.com/posts/how-to-parse-iso8601-date-in-swift/#caveats
• https://stackoverflow.com/questions/28016578/how-can-i-parse-create-a-date-time-stamp-formatted-with-fractional-seconds-utc