How to Convert UTC Datetime Strings to Local Timezones in Python?

Converting UTC Datetime Strings to Local Datetimes

Question:

How to convert a UTC datetime string, stored as a string in App Engine's Bigtable, into a datetime in the end-user's correct timezone?

Answer:

To convert a UTC datetime string into a datetime in the user's correct timezone, one can use the python-dateutil library. This library provides tzinfo implementations on top of a zoneinfo (Olson) database, allowing for easy referencing of time zone rules by canonical names.

Implementation:

from datetime import datetime
from dateutil import tz

# Hardcode zones:
from_zone = tz.gettz('UTC')
to_zone = tz.gettz('America/New_York')

# Auto-detect zones:
from_zone = tz.tzutc()
to_zone = tz.tzlocal()

# Create a datetime object from the UTC string
utc = datetime.strptime('2011-01-21 02:37:21', '%Y-%m-%d %H:%M:%S')

# Convert the datetime object to UTC timezone
utc = utc.replace(tzinfo=from_zone)

# Convert the datetime object to the user's timezone
local = utc.astimezone(to_zone)

Recommended Storage for Timezone Information:

• "-5:00" or "EST": This method is simple but limited as it assumes a constant offset for the user's timezone.
• Canonical names: Using canonical names from the Olson database is more flexible and timezone-aware, as it takes into account historical changes in time zones. The python-dateutil library allows for referencing timezones by canonical names.

For example, "-5:00" can be converted to "America/New_York" using:

import pytz
est = pytz.timezone("America/New_York")
est_name = est.zone

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