datetime模块中包含如下类：
date：日期对象，常用的属性有year，month，day；
time：时间对象；
datetime：日期时间对象，常用的属性有hour，minute，second，microsecond；
timedelta：时间间隔，即两个时间点之间的长度；
datetime基本操作：
1、获取当前datetime
>>> import datetime
>>> a = datetime.datetime.now()
2018-07-26 14:28:22 249766
2、获取当天date：
>>> import datetime
>>> a = datetime.date.today()
2018-07-26
3、获取明天/前N天
明天：
>>> import datetime
>>> a = datetime.date.today()+datetime.timedelta(days=1)
>>> print(a)
2018-07-27
前三天：
a = datetime.date.today()+datetime.timedelta(days=-3)
or
a = datetime.date.today()-datetime.timedelta(days=3)
4、获取当天开始和结束时间(00:00:00 23:59:59)
>>> import datetime
>>> a=datetime.date.today()
>>> b=datetime.time.min
>>> c=datetime.time.max
>>> print(datetime.datetime.combine(a,b))
>>> print(datetime.datetime.combine(a,c))
5、获取两个datetime的时间差
>>> (datetime.datetime(2018,7,26,15,34,0)-datetime.datetime.now()).total_seconds()
6、获取本周/本月/上月最后一天
本周：
>>> today = datetime.date.today()
>>> today
>>> sunday=today+datetime.timedelta(6-today.weekday())
7、Datetime Object/String/timestamp/time tuple关系转换
A、datetime->string
>>> import datetime
>>> datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
'2018-01-12 23:13:08'
B、string->datetime
>>> import datetime
>>> datetime.datetime.strptime("2014-12-31 18:20:10", "%Y-%m-%d %H:%M:%S")
datetime.datetime(2014, 12, 31, 18, 20, 10)
C、datetime->timetuple
>>> import datetime
>>> datetime.datetime.now().timetuple()
time.struct_time(tm_year=2015, tm_mon=1, tm_mday=12, tm_hour=23, tm_min=17, tm_sec=59, tm_wday=0, tm_yday=12, tm_isdst=-1)
D、datetime->date
>>> import datetime
>>> datetime.datetime.now().date()
datetime.date(2015, 1, 12)
E、date->datetime
>>> datetime.date.today()
datetime.date(2015, 1, 12)
>>> today = datetime.date.today()
>>>datetime.datetime.combine(today, datetime.time())
datetime.datetime(2015, 1, 12, 0, 0)
>>> datetime.datetime.combine(today, datetime.time.min)
datetime.datetime(2015, 1, 12, 0, 0)
F、datetime->timestamp
>>> now = datetime.datetime.now()
>>> timestamp = time.mktime(now.timetuple())
>>> timestamp 1421077403.0
G、timestamp->datetime
>>> datetime.datetime.fromtimestamp(1421077403.0)
datetime.datetime(2015, 1, 12, 23, 43, 23)