Parsing or formatting an RFC 3339 timestamp using NSDateFormatter

rfc3339.swift

import Foundation

/// Parse RFC 3339 date string to NSDate
///
/// :param: rfc3339DateTimeString string with format "yyyy-MM-ddTHH:mm:ssZ"
/// :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("net.kristopherjohnson.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':'ssXXX"
        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 {
    if let threadDictionary = NSThread.currentThread().threadDictionary {
        if let cachedObject = threadDictionary[key] as T? {
            return cachedObject
        }
        else {
            let newObject = create()
            threadDictionary[key] = newObject
            return newObject
        }
    }
    else {
        assert(false, "threadDictionary should never be nil")

        // If we don't have a thread-local dictionary for some reason,
        // then just call create() on every call
        return create()
    }
}

rfc3339Test.swift

import UIKit
import XCTest

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: NSGregorianCalendar)!
        cal.locale = self.en_US_POSIX
        cal.timeZone = self.utcTimeZone
        return cal
    }()

    let calendarUnits: NSCalendarUnit =
        .YearCalendarUnit | .MonthCalendarUnit | .DayCalendarUnit |
        .HourCalendarUnit | .MinuteCalendarUnit | .SecondCalendarUnit

    func testUTCStringToDate() {
        let string = "1967-01-17T16:34:25Z"
        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)
        }
    }

    func testESTExtendedStringToDate() {
        let string = "1967-01-17T11:34:25-05:00"
        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)

            let newString = rfc3339DateTimeStringForDate(date)
            XCTAssertEqual("1967-01-17T16:34:25Z", newString)
        }
    }

    func testESTBasicFormatStringToDate() {
        let string = "2001-01-02T10:43:42-0500"
        let optDate = dateForRFC3339DateTimeString(string)
        XCTAssertNotNil(optDate)
        if let date = optDate {
            let components = calendar.components(calendarUnits, fromDate: date)
            XCTAssertEqual(2001, components.year)
            XCTAssertEqual(1, components.month)
            XCTAssertEqual(2, components.day)
            XCTAssertEqual(15, components.hour)
            XCTAssertEqual(43, components.minute)
            XCTAssertEqual(42, components.second)

            let newString = rfc3339DateTimeStringForDate(date)
            XCTAssertEqual("2001-01-02T15:43:42Z", newString)
        }
    }
}