Python 日期时间库，在向日期时间添加天数时可以识别夏令时

Question

Is there a python datetime library that does this?

dt = iso8601.parse_date("2016-03-13 00:00:00-08")
dt = dt.astimezone(us_pacific)
dt += timedelta(days=1)
self.assertEqual(dt, iso8601.parse_date("2016-03-14 00:00:00-07"))

In other words, I'm looking for a library where the datetime object is smart enough to know that timedelta(days=1) is NOT the same as timedelta(hours=24) (bonus points for supporting months).

Currently, I'm finding the least bug-hazard workaround is to use dates and then convert that to a datetime with the time set to 0 in the given timezone. The issue though is that its easy to assume the code above will work as given, so I would like to work with a library where that is not a risk. Likewise, if you're given a datetime object, its a nuisance to convert back into dates.

Edit:

This was put on hold since asking if there is a software library that accomplishes a task vs. having to code it out by scratch is offtopic. I'm not sure how to better phrase, since I've already explained the workaround is to work with dates and then add the time later, which can be done in various ways. I'm looking for a BETTER way, which I already know isn't part of the python standard library. I'm pretty sure asking "is there a better way to do this?", or "how can I get accomplish this in about the same number of lines of code?" are perfectly valid questions as well.

If there's no library that does such a thing, I could certainly make it. I'd call it YADateTime because it would be yet another python date time library. That said, if there's already a library there's no point in me making one.

Answer 1

This worked for me on Python 3.6.4. The day was added correctly according to the timezone-aware datetime objects (ie the date incremented and the hour stayed at 0). The problem I had was that the timezone offset was not correct.

Not pretty, but it seems to work.

EST = pytz.timezone('US/Eastern')
DAY = dt.timedelta(days = 1)

date = date + DAY
date_with_correct_timezone = EST.localize(dt.datetime(date.year,date.month,date.day,hour,minute,0))
if date != date_with_correct_timezone:
    print(f'     corrected {date} to {date_with_correct_timezone}')
    date = date_with_correct_timezone

Example Output:

corrected 2017-03-13 00:00:00-05:00 to 2017-03-13 00:00:00-04:00
corrected 2017-11-06 00:00:00-04:00 to 2017-11-06 00:00:00-05:00

Answer 2

This is an old question, but it's a useful question, and I didn't find any newer version of it on SO, so I'm adding an answer here.

At minimum, one should be able to take a datetime of 2022-03-13 01:30:00 in the America/Chicago timezone, which is just before the DST "spring forward" from 2am to 3am, add an hour to it, and get 2022-03-13 03:30:00 in the same America/Chicago time zone.

This isn't even adding an interval of varying length, it's just crossing a DST boundary, so it should be "easy".

The following things fail that all look like they should work:

# 0a. pytz timezone, datetime.timedelta math
import pytz
from datetime import datetime, timedelta
tz = pytz.timezone('America/Chicago')
d = datetime(2022, 3, 13, 1, 30, tzinfo=tz)  # already well-known to be wrong
result = d + timedelta(hours=1)
str(result)
# '2022-03-13 02:30:00-05:51'
# Wrong ↑

# 1a. pytz timezone, datetime.timedelta math
import pytz
from datetime import datetime, timedelta
tz = pytz.timezone('America/Chicago')
d = tz.localize(datetime(2022, 3, 13, 1, 30))
result = d + timedelta(hours=1)
str(result)
# '2022-03-13 02:30:00-06:00'
# Wrong ↑

# 2a. dateutil timezone, datetime.timedelta math, method 1
import dateutil.tz
from datetime import datetime, timedelta
tz = dateutil.tz.gettz('America/Chicago')
d = datetime(2022, 3, 13, 1, 30, tzinfo=tz)
result = d + timedelta(hours=1)
str(result)
# '2022-03-13 02:30:00-05:00'
# Wrong ↑

# 3a. dateutil timezone, datetime.timedelta math, method 2
import dateutil.tz
from datetime import datetime, timedelta
tz = dateutil.tz.gettz('America/Chicago')
d = datetime(2022, 3, 13, 1, 30).astimezone(tz)
result = d + timedelta(hours=1)
str(result)
# '2022-03-13 02:30:00-05:00'
# Wrong ↑

# 4a. dateutil timezone, pendulum math
import dateutil.tz
from datetime import datetime
import pendulum
tz = dateutil.tz.gettz('America/Chicago')
d = datetime(2022, 3, 13, 1, 30).astimezone(tz)
result = pendulum.instance(d).add(hours=1)
str(result)
# '2022-03-13T02:30:00-06:00'
# Wrong ↑

Some of those can be rescued by converting the result to a timestamp (seconds since epoch GMT) and back again, and reapplying the timezone - obviously something that really should not have to be done:

# 1b. pytz timezone, datetime.timedelta math
import pytz
from datetime import datetime, timedelta
tz = pytz.timezone('America/Chicago')
d = tz.localize(datetime(2022, 3, 13, 1, 30))
result = d + timedelta(hours=1)
result = datetime.fromtimestamp(result.timestamp()).astimezone(tz)
str(result)
# '2022-03-13 03:30:00-05:00'
# Correct ↑

# 2b. dateutil timezone, datetime.timedelta math, method 1
import dateutil.tz
from datetime import datetime, timedelta
tz = dateutil.tz.gettz('America/Chicago')
d = datetime(2022, 3, 13, 1, 30, tzinfo=tz)
result = d + timedelta(hours=1)
result = datetime.fromtimestamp(result.timestamp()).astimezone(tz)
str(result)
# '2022-03-13 01:30:00-06:00'
# Wrong ↑ (worse!)

# 3b. dateutil timezone, datetime.timedelta math, method 2
import dateutil.tz
from datetime import datetime, timedelta
tz = dateutil.tz.gettz('America/Chicago')
d = datetime(2022, 3, 13, 1, 30).astimezone(tz)
result = d + timedelta(hours=1)
result = datetime.fromtimestamp(result.timestamp()).astimezone(tz)
str(result)
# '2022-03-13 01:30:00-06:00'
# Wrong ↑ (worse!)

# 4b. dateutil timezone, pendulum math
import dateutil.tz
from datetime import datetime
import pendulum
tz = dateutil.tz.gettz('America/Chicago')
d = datetime(2022, 3, 13, 1, 30).astimezone(tz)
result = pendulum.instance(d).add(hours=1)
result = datetime.fromtimestamp(result.timestamp()).astimezone(tz)
str(result)
# '2022-03-13 03:30:00-05:00'
# Correct ↑