Python – Setting a datetime in a specific timezone (without UTC conversions)

Question

Just to be clear, this is python 2.6, I am using pytz.

This is for an application that only deals with US timezones, I need to be able to anchor a date (today), and get a unix timestamp (epoch time) for 8pm and 11pm in PST only.

This is driving me crazy.

> pacific = pytz.timezone("US/Pacific")

> datetime(2011,2,11,20,0,0,0,pacific)

datetime.datetime(2011, 2, 11, 20, 0, tzinfo=<DstTzInfo 'US/Pacific' PST-1 day, 16:00:0 STD>)

> datetime(2011,2,11,20,0,0,0,pacific).strftime("%s")
'1297454400'

zsh> date -d '@1297454400'    
Fri Feb 11 12:00:00 PST 2011

So, even though I am setting up a timezone, and creating the datetime with that time zone, it is still creating it as UTC and then converting it. This is more of a problem since UTC will be a day ahead when I am trying to do the calculations.

Is there an easy (or at least sensical) way to generate a timestamp for 8pm PST today?

(to be clear, I do understand the value of using UTC in most situations, like database timestamps, or for general storage. This is not one of those situations, I specifically need a timestamp for evening in PST, and UTC should not have to enter into it.)

Asked By: liam

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Answer 1

Create a tzinfo object utc for the UTC time zone, then try this:

#XXX: WRONG (for any timezone with a non-fixed utc offset), DON'T DO IT
datetime(2011,2,11,20,0,0,0,pacific).astimezone(utc).strftime("%s")

Edit: As pointed out in the comments, putting the timezone into the datetime constructor isn’t always robust. The preferred method using the pytz documentation would be:

pacific.localize(datetime(2011,2,11,20,0,0,0)).astimezone(utc).strftime("%s")

Also note from the comments that strftime("%s") isn’t reliable, it ignores the time zone information (even UTC) and assumes the time zone of the system it’s running on. It relies on an underlying C library implementation and doesn’t work at all on some systems (e.g. Windows).

Answered By: Mark Ransom

Answer 2

There are at least two issues:

you shouldn’t pass a timezone with non-fixed UTC offset such as "US/Pacific" as tzinfo parameter directly. You should use pytz.timezone("US/Pacific").localize() method instead
.strftime('%s') is not portable, it ignores tzinfo, and it always uses the local timezone. Use datetime.timestamp() or its analogs on older Python versions instead.

To make a timezone-aware datetime in the given timezone:

#!/usr/bin/env python
from datetime import datetime 
import pytz # $ pip install pytz

tz = pytz.timezone("US/Pacific")
aware = tz.localize(datetime(2011, 2, 11, 20), is_dst=None)

To get POSIX timestamp:

timestamp = (aware - datetime(1970, 1, 1, tzinfo=pytz.utc)).total_seconds()

(On Python 2.6, see totimestamp() function on how to emulate .total_seconds() method).

Answered By: jfs

Python – Setting a datetime in a specific timezone (without UTC conversions)

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