python夏令时问题

Question

我在夏令时方面遇到了麻烦。

让我解释一下上下文。

我在数据库中存储UTC日期。每个用户都有一个时区，我也将这些信息存储在数据库中。

我根据每个用户的时区计算出"1个月"的周期。

例如，用户应该看到：

2014-01-06 -> 2014-02-05
2014-02-06 -> 2014-03-05
2014-03-06 -> 2014-04-05
2014-04-06 -> 2014-05-05
2014-05-06 -> 2014-06-05
etc.

当夏令时开启或关闭（三月和十一月）时，我遇到了一个问题

代替：

• 2014-03-06 -> 2014-04-05，我得到2014-03-06 -> 2014-04-06
• 2014-11-06 -> 2014-12-05，我得到2014-11-05 -> 2014-12-04

然后它会被所有下一个偏移。

我尝试使用UTC日期，而不是将它们转换为用户时区以在get_end_date中添加1个月，但我仍然存在问题。

知道怎么处理吗？

感谢

这是我的代码和get_end_date方法。

import pytz
from dateutil.parser import parse as parse_date
from dateutil.relativedelta import relativedelta
def to_tz(dt, tz):
    tz = pytz.timezone(tz)
    return tz.normalize(dt.astimezone(tz))
def get_end_date(dt, tz):
    dt_central = to_tz(dt, tz)
    # Add 1 month, set hours, minutes, seconds, microseconds to 0 and remove -1 ms
    dt_central = dt_central + relativedelta(months=+1)
    dt_central = dt_central.replace(hour=0, minute=0, second=0, microsecond=0)
    dt_central = dt_central + relativedelta(microseconds=-1)
    return to_tz(dt_central, 'UTC')
def print_periods(start_dt, tz, nb_periods=6):    
    for i in xrange(nb_periods):
        print 'n# Period {}'.format(i+1)
        end_dt = get_end_date(start_dt, tz)
        start_dt_central, end_dt_central = to_tz(start_dt, tz), to_tz(end_dt, tz)
        print '{} -> {}'.format(start_dt_central.date(), end_dt_central.date())
        print '  > start_dt_central: ', start_dt_central
        print '  > end_dt_central: ', end_dt_central
        print '  > start_dt: ', start_dt
        print '  > end_dt: ', end_dt
        start_dt = end_dt + relativedelta(microseconds=1)

输出：

central_tz = 'US/Central'
start_dt = parse_date('2014-01-06 06:00:00+00:00')
In [7]: print_periods(start_dt, central_tz, nb_periods=12)
# Period 1
2014-01-06 -> 2014-02-05
  > start_dt_central:  2014-01-06 00:00:00-06:00
  > end_dt_central:  2014-02-05 23:59:59.999999-06:00
# Period 2
2014-02-06 -> 2014-03-05
  > start_dt_central:  2014-02-06 00:00:00-06:00
  > end_dt_central:  2014-03-05 23:59:59.999999-06:00
# Period 3
2014-03-06 -> 2014-04-06
  > start_dt_central:  2014-03-06 00:00:00-06:00
  > end_dt_central:  2014-04-06 00:59:59.999999-05:00
# Period 4
2014-04-06 -> 2014-05-05
  > start_dt_central:  2014-04-06 01:00:00-05:00
  > end_dt_central:  2014-05-05 23:59:59.999999-05:00
# Period 5
2014-05-06 -> 2014-06-05
  > start_dt_central:  2014-05-06 00:00:00-05:00
  > end_dt_central:  2014-06-05 23:59:59.999999-05:00
# Period 6
2014-06-06 -> 2014-07-05
  > start_dt_central:  2014-06-06 00:00:00-05:00
  > end_dt_central:  2014-07-05 23:59:59.999999-05:00
# Period 7
2014-07-06 -> 2014-08-05
  > start_dt_central:  2014-07-06 00:00:00-05:00
  > end_dt_central:  2014-08-05 23:59:59.999999-05:00
# Period 8
2014-08-06 -> 2014-09-05
  > start_dt_central:  2014-08-06 00:00:00-05:00
  > end_dt_central:  2014-09-05 23:59:59.999999-05:00
# Period 9
2014-09-06 -> 2014-10-05
  > start_dt_central:  2014-09-06 00:00:00-05:00
  > end_dt_central:  2014-10-05 23:59:59.999999-05:00
# Period 10
2014-10-06 -> 2014-11-05
  > start_dt_central:  2014-10-06 00:00:00-05:00
  > end_dt_central:  2014-11-05 22:59:59.999999-06:00
# Period 11
2014-11-05 -> 2014-12-04
  > start_dt_central:  2014-11-05 23:00:00-06:00
  > end_dt_central:  2014-12-04 23:59:59.999999-06:00
# Period 12
2014-12-05 -> 2015-01-04
  > start_dt_central:  2014-12-05 00:00:00-06:00
  > end_dt_central:  2015-01-04 23:59:59.999999-06:00

更新：解决方案

Christian 的解决方案1

def get_end_date(dt, tz):
    dt_central = to_tz(dt, tz)
    orig_tz = dt_central.tzinfo
    dt_central = dt_central.replace(tzinfo=None)
    # Calculations
    dt_central = orig_tz.localize(dt_central)
    dt_utc = to_tz(dt_central, 'UTC')
    return dt_utc 

解决方案2来自我

def get_end_date(dt, tz):
    dt_central = to_tz(dt, tz)
    # Calculations
    dt_utc = to_tz(dt_central, 'UTC')
    # When switch to Daylight Saving Time
    if to_tz(dt_utc, tz).hour == 0:
        dt_utc = dt_utc.replace(hour=dt_utc.hour-1)
    # When leave Daylight Saving Time  
    elif to_tz(dt_utc, tz).hour == 22:
        dt_utc = dt_utc.replace(hour=dt_utc.hour+1)
    return dt_utc

Answer 1

在扩展vevents的reccurrence规则时，dateutil的rrule也遇到了类似的问题。对我来说，有效的方法是保存时区信息，然后将每个日期时间对象转换为天真日期时间对象，进行计算，然后用保存的时区定位每个日期时间。

因此，沿着这些路线：

orig_tz = dt.tzinfo
dt = dt.replace(tzinfo=None)
# do calculations here
orig_tz.localize(dt)