1. He has lunch at 12:00.

Question

2. He has lunch at 12:00 p.m.
3. He has lunch at 12.
4. He has lunch at 12 p.m.
5. He has lunch at noon.
(Are all grammatical?)

6. He goes back home at 15:30 p.m.
7. He goes back home at 15:30.
8. He goes back home at 3:30 p.m.
9. He goes back home at 3:30.
(Are all acceptable?)

Answer 1

Correct.

Answer 2

Most of us don't use the 24-hour clock, so 6 and 7 would not be acceptable to a lot of Americans.

Answer 3

But It would still be acceptable, just because many don't use does not mean that its not Acceptable. Its still correct.

Answer 4

I've not seen people use PM with the 24 hour clocks. Is that correct? I honestly do not know.