time zone aware timestamps in JavaScript and Python

By Jochen Voss, on 2012-07-17

For one of my programming experiments, I want to compute timestamps by using the number of seconds since the beginning of 1970. The resulting value will be used for communication between a web browser and my web server. My aim is to find a method to get the very similar values for the timestamp in JavaScript code running on the web browser and in a Python program running on my web server.

To solve this problem, I need a method which does not depend on the time zone the web user is in, and which does not break when the daylight saving time (DST) starts or ends. One solution to this problem is, to perform the computation of the timestamp in Coordinated Universal Time (UTC, a successor of the Greenwich Mean Time). For reference, this post describes what I found.

In the web browser, I am using the following JavaScript code:

  now = new Date()
  timestamp = now.getTime() / 1000

Here, the getTime method does all the work; since JavaScript timestamps use milliseconds since the beginning of 1970, we have to divide by 1000 to convert to seconds. According to the ECMAScript specification (also governing JavaScript), the resulting value should be independent of time zone and DST. For testing, I changed the clock of my computer to a different time zone; everything still worked: after restarting the browser, now reflects the new time zone, but timestamp is only changed by the few seconds I needed to restart the browser.

The code on the web server is written in Python. After a lot of experimenting, I've found two working combinations: The more straightforeward way of computing a timestamp is as follows.

  import datetime
  import time

  now = datetime.datetime.now()
  timestamp = time.mktime(now.timetuple()) + now.microsecond * 1e-6

Despite the fact that there is no explicit conversion to UTC in this code, the computed timestamp gives the number of seconds since the start of 1970 in UTC. This depends on time.mktime's ability to magically know the current time zone of the computer this code is running on. The same computation can also be done using times in UTC directly.

  import datetime
  import calendar

  now = datetime.datetime.utcnow()
  timestamp = calendar.timegm(now.utctimetuple()) + now.microsecond * 1e-6

This time, the code depends on the utcnow function knowing the current timezone. An ugly point of the second solution is, that it requires the slightly obscure calendar module from the Python standard library. Again, I tested this code by changing the time zone of the computer's clock and all worked as expected.