‘24 Hour Time’ function is for countries which use 0~23 hours, instead of AM/PM hours. For example, if you turn ‘24 Hour Time’ function on, alarm time will be displayed as ‘23:00,’ instead of ‘11:00PM.’

It’s just an extra function for countries which use 0~23 hour time system. You do not have to worry about alarm not ringing properly even if you leave this function off.