You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how may I serve you?

USER: I want to do some modifications in my existing reservation, can you help me in that?

ASSISTANT: Sure, I will help you. Please mention the name on which you have done the reservation.

USER: Sure, myself Robert Smith.

ASSISTANT: I have found your reservation, may I know your new travelling dates?

USER: Sure, my planned travel dates are Dec 01 and Dec 03.

ASSISTANT: May I know your nearby airport codes?

USER: Sure, my nearby airport codes are DEN and OAK.

ASSISTANT: Do you have any class or time preferences?

USER: Yes, I would like to start my journey for the evening flight.

ASSISTANT: Sorry, there is no flight available with your requirements.

USER: That's ok, no problem. Thank you for the information.

ASSISTANT: You are welcome, have a nice day.