How do I check if system is 12 or 24 hour?

Shouldn’t it be

if (!DateFormat.is24HourFormat(this))

You want to assign am/pm only when it is not set to 24 hour format, right?

Here is a more compact version:

if (!DateFormat.is24HourFormat(this)) {
    mHour = mCalendar.get(Calendar.HOUR_OF_DAY);
    int hourOfDay = mHour;
    if (hourOfDay >= 12) {
        views.setTextViewText(R.id.AMPM, "pm");
    } else {
        views.setTextViewText(R.id.AMPM, "am");
    }
} else {
    views.setTextViewText(R.id.AMPM, "");
}