Time and Timezones in Python

Processing of time information is often a critical part in a Python application. In this post, we are going to solve the following problem:

Given the departure time and the duration of a flight from New York to London, find the arrival time in London local time.

Departure New York: Jul 23 at 19:30, 2018

Flight time: 7h 15min
 

The final output will be

Task: Find the arrival time in London local time.

To solve this problem we need to consider time zones. And that can be a challenge. Time zones are not fixed, they change depending on political decisions. For example, during the summer in New York the time zone respected is EDT (Eastern Daylight Time), but during the winter the time zone EST (Eastern Standard Time) is followed. The situation is similar in London, during summer the BST (British Summer Time) is used and in the winter GMT (Greenwich Mean Time) is followed. When does the summer end and the winter start? Well, that’s not based on meteorological considerations, but on political decisions. And as a consequence, doing time calculation can rapidly escalate into an unmanageable mess of code.

Fortunately, in Python, we have the pytz module which makes time calculations involving time zones a breeze. Pytz can be installed with pip, e.g. in the terminal write pip install pytz.

Specifying a time in Python is a two step process, first we construct a naive datetime object, naive in the sense that it is not aware of the time zone. Next, we make the datetime object timezone aware by using the localize method in the pytz module.The code below shows the calculations with comments.

# Python 3.6.3
import datetime, pytz

# A naive datetime object is constructed by specifying the departure time
departure_time_clock_NewYork = datetime.datetime(2018,7,19,19,30,0)

# To make the datetime object time zone aware, use the localize method
tz_NewYork = pytz.timezone('America/New_York')
departure_time_clock_NewYork = tz_NewYork.localize(departure_time_clock_NewYork)

# The flight duration is
flight_time = datetime.timedelta(hours=7, minutes=15)

# The arival time in London can now be calculated
# Note, this is the arrivale time on the clock in New York

arrival_time_clock_NewYork = departure_time_clock_NewYork + flight_time

# To find the local time we first need the time zone in London
tz_London = pytz.timezone('Europe/London')

# Nex, use the astimezone method to find the local arrival time
arrival_time_clock_London = arrival_time_clock_NewYork.astimezone(tz_London)

# Print the itinerary. We here use of the strftime method
# to clean up the presentation

fmt = '%B %d, %Y %H:%M'

print("Flight BA 176 New York (JFK) to London (LHR)")
print('Depature (JFK):\t{}'.format(departure_time_clock_NewYork.strftime(fmt)))
print('Arrival (LHR):\t{}'.format(arrival_time_clock_London.strftime(fmt)))
print('*** All times are local ***')


And that is all that is needed. Without comments this code is only a few lines long.