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

Specifically, given the timezone of my server (system time perspective) and a timezone input, how do I calculate the system time as if it were in that new timezone (regardless of daylight savings, etc)?

import datetime
current_time = datetime.datetime.now() #system time

server_timezone = "US/Eastern"
new_timezone = "US/Pacific"

current_time_in_new_timezone = ???

Solution 1:

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.

Solution 2:

With Python 3.9, the standard lib has all you need: zoneinfo. pytz is not needed anymore (deprecated; -> pytz deprecation shim).

Ex:

from datetime import datetime
from zoneinfo import ZoneInfo

server_timezone = "US/Eastern"
new_timezone = "US/Pacific"

current_time = datetime.now(ZoneInfo(server_timezone)) 

# current_time_in_new_timezone = ???
current_time_in_new_timezone = current_time.astimezone(ZoneInfo(new_timezone))

That gives you for example

print(current_time.isoformat(timespec='seconds'))
# 2021-10-04T02:42:54-04:00

print(repr(current_time))
# datetime.datetime(2021, 10, 4, 2, 42, 54, 40600, tzinfo=zoneinfo.ZoneInfo(key='US/Eastern'))

print(current_time_in_new_timezone.isoformat(timespec='seconds'))
# 2021-10-03T23:42:54-07:00

print(repr(current_time_in_new_timezone))
# datetime.datetime(2021, 10, 3, 23, 42, 54, 40600, tzinfo=zoneinfo.ZoneInfo(key='US/Pacific'))

Solution 3:

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

There are two steps:

1. Create an aware datetime objects from the system time and timezone e.g., to get the current system time in the given timezone:

   #!/usr/bin/env python
   from datetime import datetime
   import pytz
   
   server_timezone = pytz.timezone("US/Eastern")
   server_time = datetime.now(server_timezone) # you could pass *tz* directly
   

   Note: datetime.now(server_timezone) works even during ambiguous times e.g., during DST transitions while server_timezone.localize(datetime.now()) may fail (50% chance).

   If you are sure that your input time exists in the server's timezone and it is unique then you could pass is_dst=None to assert that:

   server_time = server_timezone.localize(naive_time, is_dst=None)
   

   It raises an exception for invalid times.
   If it is acceptable to ignore upto a day error (though typically an error due to DST is around an hour) then you could drop is_dst parameter:

   server_time = server_timezone.normalize(server_timezone.localize(naive_time))
   

   .normalize() is called to adjust non-existing times (local time in the gap, during "spring forward" transitions). If the time zone rules haven't changed; your server shouldn't generate non-existing times. See "Can I just always set is_dst=True?"

2. Convert an aware datetime object to the target timezone tz:

   tz = pytz.timezone("US/Pacific")
   server_time_in_new_timezone = server_time.astimezone(tz)