     ///
     /// - throws: A `Generator.Error` if a valid password cannot be generated.
     /// - returns: The generated password, or throws an error if a password could not be generated.
-    @warn_unused_result
     public func passwordAtTime(time: NSTimeInterval) throws -> String {
         guard Generator.validateDigits(digits) else {
             throw Error.InvalidDigits

    public enum Error: ErrorType {
        /// The requested time is before the epoch date.
        case InvalidTime
        /// The requested period is not a positive number of seconds
        case InvalidPeriod
        /// The number of digits is either too short to be secure, or too long to compute.
        case InvalidDigits

        ///
        /// - throws: A `Generator.Error` if a valid counter cannot be calculated.
        /// - returns: The counter value needed to generate the password for the target time.
        private func counterAtTime(time: NSTimeInterval) throws -> UInt64 {
            switch self {
            case .Counter(let counter):
