While working at the airport, I often heard the expression "What is departure time?"
I also have heard the expression "What is check-out time?"
I'm positive that grammatically speaking, "when" would be accurate instead of "what". However, "what" has been used frequently enough as well. Is there a reason for this?
Perhaps just the same words being switched around but still conveying the same thought.
What is departure time? = What time is departure?
What is check-out time? = What time is check-out?
What/when is the departure time.
Perhaps it is just the usual frantic rush at places such as airports and railway stations that the shorter version is heard.
What is better than when:
What time does this train depart? is specific.
When does this train depart? is open to a wider interpretation. It might be today, or once a week, or in an hour's time.
What time is departure/checkout? are correct.