Python 남은 시간 구하기

import datetime
import math
import time
 
while True:
    datetime.datetime.now()
    start_time = datetime.datetime.now()
    end_time = datetime.datetime(2018, 5, 15)
    how_long = end_time - start_time
    days = how_long.days
    hours = how_long.seconds // 3600
    minutes = how_long.seconds // 60 - hours * 60
    seconds = how_long.seconds - hours * 3600 - minutes * 60
 
    print('{}일 {}시간 {}분 {}초 남았습니다.'.format(days, hours, minutes, seconds))
    time.sleep(1)