How to depict a timer with a Sequence diagram ?

Dear all,
I need a little help to depict a sequence diagram.

My use case is : a client issues a request to a Servlet.
The Servlet registers to a TimerListener and then starts a remote back-
end service.

If the service is not completed within "x" seconds then the
TimerListener invokes the timerExpired methods and return a "Server
busy" page to the client.

I'd like to depict this scenario with a sequence diagram but I don't
know how to handle the TimerListener event ( that can raise if the
back-end operation takes too long).

Any help ?
Thanks a lot
Laura

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