Close, but not exactly what I wanted. I happen to know sunrise tomorrow is a few minutes earlier than sunrise today (as summer progresses). So I clarify, and in the process I can see Siri's context engine at work. The next single word I say is interpreted relative to facts within the larger conversation.
Aha. She understands I'm talking about a sunrise. She gets the concept of tomorrow. She just doesn't know the answer. This is standard almanac data, something Wolfram Alpha should handle easily, but that's another story.
When Siri doesn't understand me, I usually find I'm asking for something that she's not programmed to recognize as a task. How do I know? I simply ask her.