The 24-hour clock is the most commonly used time notation in the world. That is if you're not living in the US or any of the few countries that sill operate on a 12-hour clock system. Also called military time in the US, 24-hour time format arguably makes more sense than anything else as it divides the day into -- you guessed it -- 24 hours.
How to change the Calendar app to 24-hour clock time format on Mac