I just realised that the kanji for "when", いつ, and the kanji for "what time", なんじ are the same - 何時
How are you suppose to know what to say when you see that kanji? I can understand reading things in context, but you wouldn't know whether you're suppose to say "itsu" or "nanji" until the answering sentence, which comes after the question.
For example,
Q: いつ東京へ行きますか?
A: 明日行きます。
Q: なんじに東京へ行きますか?
A: 9:00に行きます。
Hmm, maybe I just answered my own question. Has it got anything to do with the ni particle? I.e., when you see に you know that it's なんじ, or can に come after itsu as well?