python 时间差的计算

import time
import datetime

start = datetime.datetime.now()
time.sleep(30)
end = datetime.datetime.now()

print (end-start).days # 0 天数
print (end-start).total_seconds() # 30.029522 精确秒数
print (end-start).seconds # 30 秒数
print (end-start).microseconds # 29522 毫秒数

import datetime

now = datetime.datetime.now()
three_days_aGo = now + datetime.timedelta(days=-3)
three_weeks_ago = now + datetime.timedelta(weeks=-3)

print now # datetime.datetime(2016, 12, 22, 11, 24, 49, 987171)
print three_days_ago # datetime.datetime(2016, 12, 19, 11, 24, 49, 987171)
print three_weeks_ago # datetime.datetime(2016, 12, 1, 11, 24, 49, 987171)