Earlier quoted context omitted.
Yes, I'm not trying to claim that AM/PM is ambiguous, but that me as a person not used to the system (basically everyone except US and a few other countries) have no idea which is which. :)
A native Floridian once told me that the New Year's fireworks display would be starting at “12 PM”. Confused about why they’d be shooting off fireworks in the middle of the day I said, “12 PM!?” She rolled her eyes and made air quotes with her fingers and said, “Well, AM ”, as if I was being extremely pedantic. To her, AM meant morning and PM meant night. Having worked on the UI of an event-related service, I can say…
11:59pm, midnight, 12:01am
11:59am, noon, 12:01pm