如何从日期减去一天？

Question

我有一个Python datetime.datetime对象.减去一天的最佳方法是什么？

Answer 1

您可以使用timedelta对象:

from datetime import datetime, timedelta

d = datetime.today() - timedelta(days=days_to_subtract)

Answer 2

减去 datetime.timedelta(days=1)

Answer 3

如果您的Python日期时间对象是时区感知的,那么您应该小心避免DST转换周围的错误(或由于其他原因而改变UTC偏移):

from datetime import datetime, timedelta
from tzlocal import get_localzone # pip install tzlocal

DAY = timedelta(1)
local_tz = get_localzone()   # get local timezone
now = datetime.now(local_tz) # get timezone-aware datetime object
day_ago = local_tz.normalize(now - DAY) # exactly 24 hours ago, time may differ
naive = now.replace(tzinfo=None) - DAY # same time
yesterday = local_tz.localize(naive, is_dst=None) # but elapsed hours may differ

在一般情况下,day_ago以及yesterday如果UTC本地时区偏移在最后一天发生了变化可能会有所不同.

例如,夏令时/夏令时将于2014年11月2日上午02:00:00在America/Los_Angeles时区结束,因此:

import pytz # pip install pytz

local_tz = pytz.timezone('America/Los_Angeles')
now = local_tz.localize(datetime(2014, 11, 2, 10), is_dst=None)
# 2014-11-02 10:00:00 PST-0800

然后day_ago又有所yesterday不同:

• day_ago恰好是24小时前(相对于now)但是在上午11点,而不是在上午10点now
• yesterday是昨天上午10点,但它是25小时前(相对于now),而不是24小时.

pendulum模块自动处理:

>>> import pendulum  # $ pip install pendulum

>>> now = pendulum.create(2014, 11, 2, 10, tz='America/Los_Angeles')
>>> day_ago = now.subtract(hours=24)  # exactly 24 hours ago
>>> yesterday = now.subtract(days=1)  # yesterday at 10 am but it is 25 hours ago

>>> (now - day_ago).in_hours()
24
>>> (now - yesterday).in_hours()
25

>>> now
<Pendulum [2014-11-02T10:00:00-08:00]>
>>> day_ago
<Pendulum [2014-11-01T11:00:00-07:00]>
>>> yesterday
<Pendulum [2014-11-01T10:00:00-07:00]>

Answer 4

只是为了详细说明一个有用的替代方法和用例:

• 从当前日期时间减去1天:

from datetime import datetime, timedelta
print datetime.now() + timedelta(days=-1)  # Here, I am adding a negative timedelta

• 在案例中有用,如果您想要添加5天并从当前日期时间减去5小时.即从现在起5天内的日期时间是多少,但是减少5个小时？

from datetime import datetime, timedelta
print datetime.now() + timedelta(days=5, hours=-5)

它可以类似地与其他参数一起使用,例如秒,周等

Answer 5

另外,当我想要计算上个月的第一天/最后一天或其他相对时间等时,我喜欢使用另一个不错的功能......

来自dateutil函数的relativedelta函数(对datetime lib的强大扩展)

import datetime as dt
from dateutil.relativedelta import relativedelta
#get first and last day of this and last month)
today = dt.date.today()
first_day_this_month = dt.date(day=1, month=today.month, year=today.year)
last_day_last_month = first_day_this_month - relativedelta(days=1)
print (first_day_this_month, last_day_last_month)

>2015-03-01 2015-02-28

Answer 6

Genial arrow模块存在

import arrow
utc = arrow.utcnow()
utc_yesterday = utc.shift(days=-1)
print(utc, '\n', utc_yesterday)

输出:

2017-04-06T11:17:34.431397+00:00 
 2017-04-05T11:17:34.431397+00:00