Hallo! I´d like to know how to write the opening hours when something is open from noon. For morning I write 7a.m., afternoon 5p.m., but I´m not sure what shell I use in American English...12 noon??Or 12 a.m. when I mean noon?

Thank you very much for your reply in advance!!
Noon = 12 PM
Midnight = 12 AM
If you want to get rid of ambiguity, you can just say "noon" or "midnight."