You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I am William Miller.

ASSISTANT: How can I help you?

USER: I would like to visit an arts expo at Washington. So, please reserve me a flight ticket from OAK to reach IAD.

ASSISTANT: Let me know your travel dates.

USER: My planned journey dates are Dec 21 and Dec 23.

ASSISTANT: Do you have any preference?

USER: My return time should be morning.

ASSISTANT: Sorry, no flights found for your travelling dates.

USER: Ok, not an issue. Thank you for the information.

ASSISTANT: Thank you.