def timestamp(time = nil)
      @timestamp ||= begin
        time ||= Time.now.utc.freeze
        time = time.getutc.freeze unless time.utc? && time.frozen?
        time
      end
    end
def timestamp(time = nil)
      @timestamp ||= begin
        time ||= Time.now.utc.freeze
        time = time.getutc.freeze unless time.utc? && time.frozen?
        time
      end
    end
Returns the time of the current buffer as an ISO 8601 formatted string.
 If the timestamp was not yet set on the buffer, it is is set to the
 the passed `time` or the current time.

 @example
   buffer.timestamp
   # => "2016-10-17T13:37:00.234Z"
 @param time [Time] an optional time object. If no timestamp was set yet,
   this time is used
 @return [String] an ISO 8601 formatted UTC timestamp.