Python Time to Age, Part 2: Timezones

Python time to age, part 2: timezones

New in version 2.6.

For a naive object, the %z and %Z
format codes are replaced by empty
strings.

It looks like this is implemented only in >= 2.6, and I think you have to manually parse it.

I can't see another solution than to remove the time zone data:

from datetime import timedelta,datetime
try:
    offset = int("Tue, 22 Jul 2008 08:17:41 +0300"[-5:])
except:
    print "Error"

delta = timedelta(hours = offset / 100)

fmt = "%a, %d %b %Y %H:%M:%S"
time = datetime.strptime("Tue, 22 Jul 2008 08:17:41 +0200"[:-6], fmt)
time -= delta

How to preserve timezone when parsing date/time strings with strptime()?

The datetime module documentation says:

Return a datetime corresponding to date_string, parsed according to format. This is equivalent to datetime(*(time.strptime(date_string, format)[0:6])).

See that [0:6]? That gets you (year, month, day, hour, minute, second). Nothing else. No mention of timezones.

Interestingly, [Win XP SP2, Python 2.6, 2.7] passing your example to time.strptime doesn't work but if you strip off the " %Z" and the " EST" it does work. Also using "UTC" or "GMT" instead of "EST" works. "PST" and "MEZ" don't work. Puzzling.

It's worth noting this has been updated as of version 3.2 and the same documentation now also states the following:

When the %z directive is provided to the strptime() method, an aware datetime object will be produced. The tzinfo of the result will be set to a timezone instance.

Note that this doesn't work with %Z, so the case is important. See the following example:

In [1]: from datetime import datetime

In [2]: start_time = datetime.strptime('2018-04-18-17-04-30-AEST','%Y-%m-%d-%H-%M-%S-%Z')

In [3]: print("TZ NAME: {tz}".format(tz=start_time.tzname()))
TZ NAME: None

In [4]: start_time = datetime.strptime('2018-04-18-17-04-30-+1000','%Y-%m-%d-%H-%M-%S-%z')

In [5]: print("TZ NAME: {tz}".format(tz=start_time.tzname()))
TZ NAME: UTC+10:00

Python time to age

You need to use the module datetime and the object datetime.timedelta

from datetime import datetime

t1 = datetime.strptime("Mon, 17 Nov 2008 01:45:32 +0200","%a, %d %b %Y %H:%M:%S +0200")
t2 = datetime.now()

tdelta = t2 - t1 # actually a datetime.timedelta object
print tdelta.days

How do you convert a datetime/timestamp from one timezone to another timezone?

If you know your origin timezone and the new timezone that you want to convert it to, it turns out to be very straightforward:

1. Make two pytz.timezone objects, one for the current timezone and one for the new timezone e.g. pytz.timezone("US/Pacific"). You can find a list of all official timezones in pytz library: import pytz; pytz.all_timezones

2. Localize the datetime/timestamp of interest to the current timezone e.g.

current_timezone = pytz.timezone("US/Eastern")
localized_timestamp = current_timezone.localize(timestamp)

3. Convert to new timezone using .astimezone() on the newly localized datetime/timestamp from step 2 with the desired timezone's pytz object as input e.g. localized_timestamp.astimezone(new_timezone).

Done!

As a full example:

import datetime
import pytz

# a timestamp I'd like to convert
my_timestamp = datetime.datetime.now()

# create both timezone objects
old_timezone = pytz.timezone("US/Eastern")
new_timezone = pytz.timezone("US/Pacific")

# two-step process
localized_timestamp = old_timezone.localize(my_timestamp)
new_timezone_timestamp = localized_timestamp.astimezone(new_timezone)

# or alternatively, as an one-liner
new_timezone_timestamp = old_timezone.localize(my_timestamp).astimezone(new_timezone) 

Bonus: but if all you need is the current time in a specific timezone, you can conveniently pass that timezone directly into datetime.now() to get the current times directly:

datetime.datetime.now(new_timezone)

When it comes to needing timezones conversions generally, I would strongly advise that one should store all timestamps in your database in UTC, which has no daylight savings time (DST) transition. And as a good practice, one should always choose to enable time zone support (even if your users are all in a single time zone!). This will help you avoid the DST transition problem that plagues so much software today.

Beyond DST, time in software can be generally quite tricky. To get a sense of just how difficult it is to deal with time in software in general, here is a potentially enlightening resource: http://yourcalendricalfallacyis.com

Even a seemingly simple operation as converting a datetime/timestamp into a date can become non-obvious. As this helpful documentation points out:

A datetime represents a point in time. It’s absolute: it doesn’t depend on anything. On the contrary, a date is a calendaring concept. It’s a period of time whose bounds depend on the time zone in which the date is considered. As you can see, these two concepts are fundamentally different.

Understanding this difference is a key step towards avoiding time-based bugs. Good luck.

How to parse dates with -0400 timezone string in Python?

You can use the parse function from dateutil:

>>> from dateutil.parser import parse
>>> d = parse('2009/05/13 19:19:30 -0400')
>>> d
datetime.datetime(2009, 5, 13, 19, 19, 30, tzinfo=tzoffset(None, -14400))

This way you obtain a datetime object you can then use.

As answered, dateutil2.0 is written for Python 3.0 and does not work with Python 2.x. For Python 2.x dateutil1.5 needs to be used.

How to convert a UTC datetime to a local datetime using only standard library?

I think I figured it out: computes number of seconds since epoch, then converts to a local timzeone using time.localtime, and then converts the time struct back into a datetime...

EPOCH_DATETIME = datetime.datetime(1970,1,1)
SECONDS_PER_DAY = 24*60*60

def utc_to_local_datetime( utc_datetime ):
    delta = utc_datetime - EPOCH_DATETIME
    utc_epoch = SECONDS_PER_DAY * delta.days + delta.seconds
    time_struct = time.localtime( utc_epoch )
    dt_args = time_struct[:6] + (delta.microseconds,)
    return datetime.datetime( *dt_args )

It applies the summer/winter DST correctly:

>>> utc_to_local_datetime( datetime.datetime(2010, 6, 6, 17, 29, 7, 730000) )
datetime.datetime(2010, 6, 6, 19, 29, 7, 730000)
>>> utc_to_local_datetime( datetime.datetime(2010, 12, 6, 17, 29, 7, 730000) )
datetime.datetime(2010, 12, 6, 18, 29, 7, 730000)

How to convert time zone of datetime column in pandas, then remove time zone?

Just add a last step of .tz_localize(None):

import pandas as pd
d = pd.Series(['2018-11-15 19:57:55', '2018-11-15 19:59:46'])
d = pd.to_datetime(d)
d
0   2018-11-15 19:57:55
1   2018-11-15 19:59:46
dtype: datetime64[ns]

d_pacific_tz_aware = d.dt.tz_localize("GMT").dt.tz_convert('America/Los_Angeles')
d_pacific_tz_aware
0   2018-11-15 11:57:55-08:00
1   2018-11-15 11:59:46-08:00
dtype: datetime64[ns, America/Los_Angeles]

d_pacific_tz_naive = d.dt.tz_localize("GMT").dt.tz_convert('America/Los_Angeles').dt.tz_localize(None)
d_pacific_tz_naive
0   2018-11-15 11:57:55
1   2018-11-15 11:59:46
dtype: datetime64[ns]

Parsing date with timezone from an email?

email.utils has a parsedate() function for the RFC 2822 format, which as far as I know is not deprecated.

>>> import email.utils
>>> import time
>>> import datetime
>>> email.utils.parsedate('Mon, 16 Nov 2009 13:32:02 +0100')
(2009, 11, 16, 13, 32, 2, 0, 1, -1)
>>> time.mktime((2009, 11, 16, 13, 32, 2, 0, 1, -1))
1258378322.0
>>> datetime.datetime.fromtimestamp(1258378322.0)
datetime.datetime(2009, 11, 16, 13, 32, 2)

Please note, however, that the parsedate method does not take into account the time zone and time.mktime always expects a local time tuple.

>>> (time.mktime(email.utils.parsedate('Mon, 16 Nov 2009 13:32:02 +0900')) ==
... time.mktime(email.utils.parsedate('Mon, 16 Nov 2009 13:32:02 +0100'))
True

So you'll still need to parse out the time zone and take into account the local time difference, too:

>>> REMOTE_TIME_ZONE_OFFSET = +9 * 60 * 60
>>> (time.mktime(email.utils.parsedate('Mon, 16 Nov 2009 13:32:02 +0900')) +
... time.timezone - REMOTE_TIME_ZONE_OFFSET)
1258410122.0

Converting string with UTC offset to a datetime object

It looks as if strptime doesn't always support %z. Python appears to just call the C function, and strptime doesn't support %z on your platform.

Note: from Python 3.2 onwards it will always work.

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