如何获得给定时区的"午夜"UTC时间？

Question

我现在能想到的最好的就是这种怪异:

>>> datetime.utcnow() \
...   .replace(tzinfo=pytz.UTC) \
...   .astimezone(pytz.timezone("Australia/Melbourne")) \
...   .replace(hour=0,minute=0,second=0,microsecond=0) \
...   .astimezone(pytz.UTC) \
...   .replace(tzinfo=None)
datetime.datetime(2008, 12, 16, 13, 0)

即,用英语,获取当前时间(以UTC为单位),将其转换为其他时区,将时间设置为午夜,然后转换回UTC.

我不只是使用now()或localtime(),因为它会使用服务器的时区,而不是用户的时区.

我不禁感到我错过了什么,有什么想法吗？

Answer 1

我认为如果你这样做,你可以减少一些方法调用:

>>> from datetime import datetime
>>> datetime.now(pytz.timezone("Australia/Melbourne")) \
            .replace(hour=0, minute=0, second=0, microsecond=0) \
            .astimezone(pytz.utc)

但是......代码中存在一个比美学更大的问题:它会在切换到夏令时或从夏令时转换到错误的结果.

原因是日期时间构造函数既不replace()考虑DST更改也不考虑.

例如:

>>> now = datetime(2012, 4, 1, 5, 0, 0, 0, tzinfo=pytz.timezone("Australia/Melbourne"))
>>> print now
2012-04-01 05:00:00+10:00
>>> print now.replace(hour=0)
2012-04-01 00:00:00+10:00 # wrong! midnight was at 2012-04-01 00:00:00+11:00
>>> print datetime(2012, 3, 1, 0, 0, 0, 0, tzinfo=tz)
2012-03-01 00:00:00+10:00 # wrong again!

但是,tz.localize()州的文件:

此方法应该用于构造localtimes,而不是将tzinfo参数传递给datetime构造函数.

因此,您的问题就像这样解决:

>>> import pytz
>>> from datetime import datetime, date, time

>>> tz = pytz.timezone("Australia/Melbourne")
>>> the_date = date(2012, 4, 1) # use date.today() here

>>> midnight_without_tzinfo = datetime.combine(the_date, time())
>>> print midnight_without_tzinfo
2012-04-01 00:00:00

>>> midnight_with_tzinfo = tz.localize(midnight_without_tzinfo)
>>> print midnight_with_tzinfo
2012-04-01 00:00:00+11:00

>>> print midnight_with_tzinfo.astimezone(pytz.utc)
2012-03-31 13:00:00+00:00

但不保证1582年之前的日期.

Answer 2

@ hop的答案在夏令时(DST)过渡当天是错误的,例如2012年4月1日.要修复它tz.localize(),可以使用:

tz = pytz.timezone("Australia/Melbourne")
today = datetime.now(tz).date()
midnight = tz.localize(datetime.combine(today, time(0, 0)), is_dst=None)
utc_dt = midnight.astimezone(pytz.utc)        

评论相同:

#!/usr/bin/env python
from datetime import datetime, time
import pytz # pip instal pytz

tz = pytz.timezone("Australia/Melbourne") # choose timezone

# 1. get correct date for the midnight using given timezone.
today = datetime.now(tz).date()

# 2. get midnight in the correct timezone (taking into account DST)
#NOTE: tzinfo=None and tz.localize()
# assert that there is no dst transition at midnight (`is_dst=None`)
midnight = tz.localize(datetime.combine(today, time(0, 0)), is_dst=None)

# 3. convert to UTC (no need to call `utc.normalize()` due to UTC has no 
#    DST transitions)
fmt = '%Y-%m-%d %H:%M:%S %Z%z'
print midnight.astimezone(pytz.utc).strftime(fmt)