DateUtils.swift

struct DateUtils {
    static let isoDateFormatter: NSDateFormatter = NSDateFormatter()

    static func toISO8601(date: NSDate) -> String {
        isoDateFormatter.locale = NSLocale(localeIdentifier: "en_US_POSIX") // Why en_US_POSIX? - https://developer.apple.com/library/mac/qa/qa1480/
        isoDateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ" // ISO8601 format

        return isoDateFormatter.stringFromDate(date)
    }
}