It's indecipherable to most people in the U.S. who refer to it as "military time".
elif time.hour == 12 && time.ampm == PM: return 12
elif time.ampm == PM: return time.hour+12
else: return time.hour
People generally aren't used to seeing time expressed in a 24 hour format. Most would choose a clock that shows the familiar 12 hour format over a 24 hour one that requires them to subtract 12 to get the time.
I think it would be similar to selling a Fahrenheit thermometer in Europe. Most people could learn to do the conversion, but they would probably just buy a Celsius thermometer instead.