def wake_deadline(start_time, timeout)
      timeout = process_timeout(timeout)
      deadline = start_time + timeout if timeout
    end
def wake_deadline(start_time, timeout)
      timeout = process_timeout(timeout)
      deadline = start_time + timeout if timeout
    end
Calculate the desired time to wake up.
 @api private
 @param start_time [nil,Time] time when the thread is put to sleep
 @param timeout [Numeric] desired time to sleep in seconds, +nil+ for forever
 @raise [TypeError] if +start_time+ is not +nil+ or a +Numeric+
 @raise [ArgumentError] if +start_time+ is negative
 @return [Time]