You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how can I assist you today?

USER: I am Barbara Walker. Actually I want to meet my old friend's.Can you help me to book a ticket?

ASSISTANT: That's great Walker, may I know your preferable travelling dates?

USER: My source and destination is AZ-PHX to DC-DCA. My preferable dates are 01/18 to 10/20.

ASSISTANT: Sorry, there are no flights available as per your requested dates.

USER: It's OK.Thank you.

ASSISTANT: Thank you.