Fast C-based ISO8601 date parser written in Swift

NSDate+ISO8601.swift

import Foundation

extension NSDate {
    
    class func dateFromISO8601String(string: String?) -> NSDate? {
        guard let string = string else {
            return nil
        }
        
        var tm = Darwin.tm()
        
        strptime(string.cStringUsingEncoding(NSUTF8StringEncoding)!, "%Y-%m-%dT%H:%M:%S%z", &tm)
        tm.tm_isdst = -1
        let t = mktime(&tm)
        
        return NSDate(timeIntervalSince1970: NSTimeInterval(t + NSTimeZone.localTimeZone().secondsFromGMT))
    }
    
    var ISO8601String: String? {
        var buffer = CChar(80)
        
        var rawtime = Int(timeIntervalSince1970) - NSTimeZone.localTimeZone().secondsFromGMT
        let timeinfo = localtime(&rawtime)
        
        strftime(&buffer, 80, "%Y-%m-%dT%H:%M:%S%z", timeinfo)
        
        return String(CString: &buffer, encoding: NSUTF8StringEncoding)
    }
}

let aDate = NSDate.dateFromISO8601String("2016-03-06T06:36:09Z")
let aString = aDate?.ISO8601String

Thank you, sir

DateFormatter never again. Thank you!

Rather than convert to local time and then back to GMT, use this in dateFromISO8601String:

let t = timegm(&tm)
return Date(timeIntervalSince1970: TimeInterval(t))