TimeStringToDate.swift

import Foundation

/// converts a string representing an unlocalized date with fixed-format to NSDate using Unix methods (suggested in Apple's Date Formatting guide: https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/DataFormatting/Articles/dfDateFormatting10_4.html)
public func timeStringToDate(timeString: NSString, formatString: NSString) -> NSDate {
    var t = tm()
    return withUnsafeMutablePointer(&t) { t -> NSDate in
        strptime_l(timeString.UTF8String, formatString.UTF8String, t, nil)
        return NSDate(timeIntervalSince1970: NSTimeInterval(mktime(t)))
    }
}

/// converts a string representing a date in the common ISO8601 format to NSDate
public func iso8601DateStringToDate(var iso8601DateString: String) -> NSDate? {
    if count(iso8601DateString) == 25 &&
        // remove extra ':' in timezone specification
        iso8601DateString.removeAtIndex(advance(iso8601DateString.endIndex, -3)) == ":" {
        return timeStringToDate(iso8601DateString, "%Y-%m-%dT%H:%M:%S%z")
    }
    return nil
}