It's not technically possible to precisely calculate the moon's phase based on time alone. It's an optical effect that is influenced by parallax, so you have to pay attention to location as well. This is, for example, why Eid al-Adha falls on different days in different parts of the world. So the function signature itself is potentially wrong, depending on my needs. I might find that out if I had to do some Googling to finish the function, but (assuming I didn't already know) I'm not sure if that possibility would ever have occurred to me if I were using Copilot.
Copilot can spit out code that's influenced by what others have written. But can it clue you into design considerations like this? Or should we be worried that it is helping us to write code that does the wrong thing with a higher degree of confidence?