如何添加每周timedeltas关于夏令时区 [英] How to add weekly timedeltas with regards to daylight saving timezones

问题描述

我想对本地化的datetime对象添加或减去周（或几天或几个月或几年）。问题是，天真的方法将导致夏令时间区内的1小时班次。

I want to add or subtract weeks (or days or month or years) to localized datetime objects. The problem is, that the naive approach will result in 1 hour shifts due to daylight saving timezones.

2014-03-27 12:00从冬天开始之前到夏天如果我在这个时间段内添加一周的timedelta本地化在例如时区欧洲/柏林，结果将是2014-04-03 13:00。我想要有同样的时间，2014-04-03 12:00。我找到一个解决方案：

2014-03-27 12:00 is right before the switch from winter to summer time. If I add a timedelta of one week to this date localized in timezone Europe/Berlin for example, the result will be 2014-04-03 13:00. I would like to have the same hour of day, 2014-04-03 12:00. I found a solution:

from datetime import datetime, timedelta
import pytz
my_tz = pytz.timezone("Europe/Berlin")

def add_relativedelta(date, delta):
    """
    Adds the given timedelta to the given date. Shifts in timezone offsets
    will be removed.
    """
    tz = date.tzinfo
    result = tz.normalize(date + delta)
    if result.utcoffset() != date.utcoffset():
        result = tz.normalize(date.utcoffset() - result.utcoffset() + result)
    return result

date = my_tz.localize(datetime(year=2014, month=3, day=27, hour=12, minute=0))
print """{} Original localized date (winter time)
{} One week later (summer time)
{} Date one week later preserving hour of day (summer time)""".format(date,
                     my_tz.normalize(date + timedelta(days=7)),
                     add_relativedelta(date, timedelta(days=7)))


2014-03-27 12:00:00+01:00 Original localized date (winter time)
2014-04-03 13:00:00+02:00 One week later (summer time)
2014-04-03 12:00:00+02:00 Date one week later preserving hour of day (summer time)

我想知道是否有更多的通用/更好的解决方案。有什么图书馆可以解决这个问题吗？这似乎是一个很常见的问题。

I was wondering if there is more generic/better solution. Are there any libraries that could solve this? This seems to be a pretty common issue.

推荐答案

timedelta（days = 7）表示7天，如 7 * 24 小时 - 不是太阳日。
如果您将时区添加7天添加到时区，您将获得7天之后的日期时间，而不管时区中的日期时间表示 。

timedelta(days=7) means 7 days, as in 7*24 hours - not "solar days". If you add 7 days to a timezone-aware datetime, you'll obtain a datetime that is 7 days later - independently of how that datetime is represented in the timezone.

似乎您真正想要的是将delta增加到您指定的时间，忽略时区详细信息。注意区别：

It seems what you really want is to apply the delta to the time you specified, ignoring timezone details. Notice the difference:

In [13]: print my_tz.normalize( my_tz.localize( dt ) + delta )
2014-04-03 13:00:00+02:00

In [14]: print my_tz.normalize( my_tz.localize( dt + delta ) )
2014-04-03 12:00:00+02:00

所以，如果可能，将三角形应用到数据时间在他们本地化之前。

So, if possible, apply the deltas to the datetimes before they are localized.

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