Today: today = datetime.date.today ()
昨天：yesterday = today - datetime.timedelta(days=1)
上个月：last_month = today.month - 1 if today.month - 1 else 12
Current timestamp: time_stamp = time.time ()
Various types of conversion
时间戳转datetime：datetime.datetime.fromtimestamp(time_stamp)
datetime turn timestamp: int (time.mktime (today.timetuple ()))
datetime转字符串：today_str = today.strftime("%Y-%m-%d")
字符串转datetime：today = datetime.datetime.strptime(today_str,"%Y-%m-%d")
Complement difference: today + datetime.timedelta (hours = 8)