When I record calendar events on iPhone or iPad i can use a 24hour basis but not on my Macbook How can I change macbook calendar entries to 24 hour 'clock'

This solution only changes the way the time is displayed on the computer. It does not change the way I am forced into recording events on the calendar, which remain in am/pm mode.

Your 'box' is exactly what I want but when I enter the 24 hour clock option as you describe the time in the menu bar (top of screen) changes but not the way the time displays in the calendar.Even when I make a new entry in 24 hour format (e.g. John at 1300) it gets recorded as 1pm on the calendar. As I said in my original message the Apple support in this country (South Africa) seem to have given up on answering the the question. This support is franchised out so is not a direct point of contact with the Apple company. Is there some mechanism to contact Apple and ask if my macBook software is faulty in some way?