import Foundation

func deserializeWCFDateString(json:AnyObject?) -> NSDate? {

    if let jsonDateString = json as? String {

        if countElements(jsonDateString) < 18 {
            return nil
        }

        let start = advance(jsonDateString.startIndex, 6)
        let end = advance(jsonDateString.startIndex, 18)
        let text = jsonDateString.substringWithRange(start...end)

        let msSince1970 = (text as NSString).doubleValue
        let secSince1970:NSTimeInterval = msSince1970 / 1000

        return NSDate(timeIntervalSince1970: secSince1970)
    }

    return nil
}

deserializeWCFDateString("/Date(1424684197910)/")
deserializeWCFDateString("")
deserializeWCFDateString(nil)
deserializeWCFDateString(NSNull())
deserializeWCFDateString("a bunch of garbage that is long enough")