"I wake up at nine o'clock every morning."
You may feel funny when your read below.
- I wake up every morning at 9.
In English, it still looks acceptable. But in Japanese, no way.
We feel the flow from "に" followed by a verb. If something among "に" and a verb, we feel strange.
Though, you can put adverbs between them.
Its not required, but adding it wouldn't be wrong, it just might come across a little clunky or unintentional. は does wonders to bring extra special attention to what comes before it. If you did 毎朝は, it means "EVERY morning, ..." providing extra emphasis on the "every morning" portion, whereas 毎朝 on its own is just "oh yeah, by the way, in the mornings, i happen to..." casual.
This rule holds for vague times within Japanese, such as 今日 (きょう - today), 明日 (あした - tomorrow), 今 (いま - now), etc... There's no need for は or に.
If you use an absolute time, a particle is required: 九時 (くじ - 9 o'clock), 四時半 (よじはん - 4:30), etc...
I believe that you can turn an absolute time into a relative time by adding 頃 (ごろ) or 位 (ぐらい/くらい) - 九時ごろ (about 9 o'clock), 九時位 (about 9 o'cock). Once you do this, it no longer requires は or に.
No, when telling time 9 is indeed pronounced く with 時 (and 九時 is the first thing that comes up on my IME when typing くじ, while typing "きゅうじ or "きゅじ" it doesn't even come up in the top 20 options)
maybe it is because words have the same meaning that are duplicated. Imagine you can express "毎朝午後九時" There is no 9 pm in the morning.
In addition, we often say "朝９時. Instead of 午前９時.
午前９時 is too polite to use in casual situations and usual business either.
You can say 毎日朝９時〜 毎日午前９時〜 Both are grammatically correct but the latter one is no natural.