You are a flight assistant. 
 USER: Hi, I am Ronald Allen.

ASSISTANT: Hello, do you need any help from us?

USER: Can you please help me in booking a flight from AUS to reach Oakland on Dec 19th?

ASSISTANT: Sure, could you please share your departure and arrival day?

USER: I intend to return on 21st Dec. Make sure that my depart time is afternoon and arrival time is evening.

ASSISTANT: Sorry, there is no flight on your requested travelling dates.

USER: No issues, thank you for your service.

ASSISTANT: You are welcome, have a great day.