How to process timezone in Python

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When Amy makes a comment in Taiwan at 20:00(GMT+8), his friend Tom in Japan should sees the comment is at at 21:00(GMT+9).

Best Practice:

  1. Please remember using datetime.datetime.utcnow() to express object’s created time
  2. Recommend that pytz is awesome libary to handle timezone

In general, timezone is made from system time according to region of the globe.

dateime object difference between built-in library and Django

Note that time.time() and datetime.datetime.now() these functions are had adding time offset of timezone but without timezone information.

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import time
import datetime
t = time.time()
dt = datetime.datetime.fromtimestamp(t)
dt
# datetime.datetime(2018, 5, 11, 15, 11, 49, 535299)

dt = datetime.datetime.now()
dt
# datetime.datetime(2018, 5, 11, 15, 11, 49, 535299)

If you are developing in Django, the timezone can definitely show tzinfo. That can be convenience to get datetime object with tzinfo and compare the difference.

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from django.utils import timezone

now = timezone.now()
now
# datetime.datetime(2018, 5, 11, 7, 44, 9, 147444, tzinfo=<UTC>)

In the above examples, I am located in GMT+8 of timezone, the UTC dateimte object adding timezone offset equals to the datetime generated by Python built-in library without timezone.

pytz is your friend

Expressing correct datetime object with tzinfo

In order to properly expressing tzinfo of datetime generated by Python built-in library, you can mark the datetime object using pytz.

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import datetime
import pytz
tp = pytz.timezone('Asia/Taipei')
dt = datetime.datetime.now().replace(tzinfo=tp)
dt
# datetime.datetime(2018, 5, 11, 16, 27, 23, 46918, tzinfo=<DstTzInfo 'Asia/Taipei' LMT+8:06:00 STD>)

Using astimezone to adjust datetime

There is a function that is astimezone in datetime object, it can adjust datetime according to the new tzinfo.

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dt.astimezone(pytz.utc)
datetime.datetime(2018, 5, 11, 8, 21, 23, 46918, tzinfo=<UTC>)

Storing in Database

If you are developing an web application with database, please keep to the rule: storing datetime must be from UTC timezone. When you have many web application server and deploy in different region, it should consider that creating a object with a timestamp must avoid to affecting by timezone, otherwise the object shows in other region and adjusts by timezone of the region, the datetime expression will absolutely make a mistake.