2.6. Datetime Timezone

2.6.1. Rationale

• Always keep dates and times only in UTC (important!)

• Datetimes should be converted to local time only when displaying to user

• Computerphile Time & Time Zones [VideoComputerphileTimeZones]

• datetime.utcnow() - produces timezone naive date!

Figure 2.1. Comparing datetime works only when all has the same timezone (UTC)

Timezone naive datetimes:

from datetime import datetime


datetime(1957, 10, 4, 19, 28, 34)
# datetime.datetime(1957, 10, 4, 19, 28, 34)

datetime.now()
# datetime.datetime(1957, 10, 4, 19, 28, 34)

datetime.utcnow()
# datetime.datetime(1957, 10, 4, 17, 28, 34)

Timezone aware datetime:

from datetime import datetime, timezone


datetime.now(tz=timezone.utc)
# datetime.datetime(1957, 10, 4, 19, 28, 34, tzinfo=datetime.timezone.utc)

datetime(1957, 10, 4, 19, 28, 34, tzinfo=timezone.utc)
# datetime.datetime(1957, 10, 4, 19, 28, 34, tzinfo=datetime.timezone.utc)

dt = datetime(1957, 10, 4, 19, 28, 34)
dt.replace(tzinfo=timezone.utc)
# datetime.datetime(1957, 10, 4, 19, 28, 34, tzinfo=datetime.timezone.utc)

datetime.utcnow(tz=timezone.utc)
# Traceback (most recent call last):
# TypeError: utcnow() takes no keyword arguments

datetime.utcnow(timezone.utc)
# Traceback (most recent call last):
# TypeError: utcnow() takes no arguments (1 given)

2.6.2. IANA Time Zone Database

• https://en.wikipedia.org/wiki/List_of_tz_database_time_zones#List

• https://www.iana.org/time-zones

2.6.3. Standard Library

• Since Python 3.9: PEP 615 -- Support for the IANA Time Zone Database in the Standard Library

from zoneinfo import ZoneInfo
from datetime import datetime, timedelta


dt = datetime(2020, 10, 31, 12, tzinfo=ZoneInfo("America/Los_Angeles"))  # Daylight saving time
print(dt)
# 2020-10-31 12:00:00-07:00
dt.tzname()
# 'PDT'


dt += timedelta(days=7)  # Standard time
print(dt)
# 2020-11-07 12:00:00-08:00
print(dt.tzname())
# PST

2.6.4. pytz

pytz brings the Olson tz database into Python:

from pytz import timezone


timezone('UTC')
timezone('US/Eastern')
timezone('Europe/Warsaw')
timezone('Asia/Almaty')

From naive to local time:

from datetime import datetime
from pytz import timezone


my_date = datetime(1969, 7, 21, 2, 56, 15)

timezone('UTC').localize(my_date)
# datetime.datetime(1969, 7, 21, 2, 56, 15, tzinfo=<UTC>)

From naive to local time:

from datetime import datetime
from pytz import timezone


my_date = datetime(1961, 4, 12, 6, 7)

timezone('Asia/Almaty').localize(my_date)
# datetime.datetime(1961, 4, 12, 6, 7, tzinfo=<DstTzInfo 'Asia/Almaty' +06+6:00:00 STD>)

From UTC to local time:

from datetime import datetime
from pytz import timezone


my_date = datetime(1969, 7, 21, 2, 56, 15, tzinfo=timezone('UTC'))

my_date.astimezone(timezone('Europe/Warsaw'))
# datetime.datetime(1969, 7, 21, 3, 56, 15, tzinfo=<DstTzInfo 'Europe/Warsaw' CET+1:00:00 STD>)

Between timezones:

from datetime import datetime
from pytz import timezone


my_date = datetime(1961, 4, 12, 6, 7, tzinfo=timezone('Asia/Almaty'))

my_date.astimezone(timezone('Europe/Warsaw'))
# datetime.datetime(1961, 4, 12, 1, 59, tzinfo=<DstTzInfo 'Europe/Warsaw' CET+1:00:00 STD>)

Descriptor Timezone Converter:

from dataclasses import dataclass
from datetime import datetime
from pytz import timezone


class Timezone:
    def __init__(self, name):
        self.timezone = timezone(name)

    def __get__(self, parent, *args):
        return parent.utc.astimezone(self.timezone)

    def __set__(self, parent, new_datetime):
        local_time = self.timezone.localize(new_datetime)
        parent.utc = local_time.astimezone(timezone('UTC'))


@dataclass
class Time:
    utc = datetime.now(tz=timezone('UTC'))
    warsaw = Timezone('Europe/Warsaw')
    moscow = Timezone('Europe/Moscow')
    est = Timezone('America/New_York')
    pdt = Timezone('America/Los_Angeles')


t = Time()

print('Launch of a first man to space:')
t.moscow = datetime(1961, 4, 12, 9, 6, 59)
print(t.utc)        # 1961-04-12 06:06:59+00:00
print(t.warsaw)     # 1961-04-12 07:06:59+01:00
print(t.moscow)     # 1961-04-12 09:06:59+03:00
print(t.est)        # 1961-04-12 01:06:59-05:00
print(t.pdt)        # 1961-04-11 22:06:59-08:00

print('First man set foot on a Moon:')
t.warsaw = datetime(1969, 7, 21, 3, 56, 15)
print(t.utc)        # 1969-07-21 02:56:15+00:00
print(t.warsaw)     # 1969-07-21 03:56:15+01:00
print(t.moscow)     # 1969-07-21 05:56:15+03:00
print(t.est)        # 1969-07-20 22:56:15-04:00
print(t.pdt)        # 1969-07-20 19:56:15-07:00

2.6.5. Assignments

Code 2.9. Solution

"""
* Assignment: Datetime Timezone Convert
* Complexity: easy
* Lines of code: 5 lines
* Time: 13 min

English:
    1. Use data from "Given" section (see below)
    2. Convert given date to `datetime` objects
    3. What was the time in:
        a. London, United Kingdom
        b. Moscow, Russian Federation
        c. Warsaw, Poland
        d. Tokyo, Japan
        e. Sydney, Australia
        f. Auckland, New Zealand

Polish:
    1. Użyj danych z sekcji "Given" (patrz poniżej)
    2. Przekonwertuj podaną datę do obiektu `datetime`
    3. Wyświetl datę jaka była w:
        a. London, Wielka Brytania
        b. Moscow, Rosja
        c. Warsaw, Polska
        d. Tokyo, Japan
        e. Sydney, Australia
        f. Auckland, Nowa Zelandia

Extra Task:
    1. Kosmodrom Bajkonur, Kazachstan
    2. Cape Canaveral, FL, USA
    3. Houston, TX, USA
    4. New York, USA
    5. South Pole
    6. North Pole

Tests:
    TODO: Doctests
    TODO: Make solution to the new assignment description
"""


# Given
from datetime import datetime
from pytz import timezone


DATA = '1969-07-21 02:56:15 UTC'