时间不是能随便加减，要用到datetime内部的datetime.timedelta()
用下面代码方便准确的获取昨天，上个小时，上个月。
import datetime
today = datetime.datetime.now().day
yesterday = (datetime.datetime.now() - datetime.timedelta(days=1)).day
hour = datetime.datetime.now().hour
last_hour = (datetime.datetime.now() - datetime.timedelta(minutes=60)).hour
month = datetime.datetime.now().replace(day=1).month
last_month = (datetime.datetime.now().replace(day=1) - datetime.timedelta(days=1)).month
