def hh_mm(time_struct, twelve_hour=True):
    """ Given a time.struct_time, return a string as H:MM or HH:MM, either
        12- or 24-hour style depending on twelve_hour flag.
    """
    postfix = ""
    if twelve_hour:
        if time_struct.tm_hour > 12:
            hour_string = str(time_struct.tm_hour - 12) # 13-23 -> 1-11 (pm)
            postfix = "pm"
        elif time_struct.tm_hour > 0:
            hour_string = str(time_struct.tm_hour) # 1-12
            postfix = "am"
        else:
            hour_string = '12' # 0 -> 12 (am)
            postfix = "pm"
    else:
        hour_string = '{hh:02d}'.format(hh=time_struct.tm_hour)
    return hour_string + ':{mm:02d}'.format(mm=time_struct.tm_min) + postfix