This form is rare in the U.S., outside the military and a few other specialized areas. If you said "Let's have dinner at nineteen," people would think you are strange (or European, if they had some experience with European time descriptions and detected an accent in your speech)....Would it be possible to say 19:00?
Neither I nor the style guide used by virtually all U.S. newspapers accede to the use of the wordy and ambiguous "o'clock."When you want to say 7 o' clock, but in the evening, not in the morning, how do you say that?
7 o clock pm is fine?
Would it be possible to say 19:00?
However, just to prove that we Yanks are an inconsistent lot, American Airlines uses a 12-hour clock with "AM/PM," and Delta uses a 12-hour clock with standard type for AM and boldface type for PM. (Both the PDF timetables I just checked are a few months old; either or both may have changed since.)...For example, United Airlines (either the largest or second-largest U.S. carrier) issues all its flight schedules on the 24-hour clock.