1. Time zone database
Now that you understand how UTC offsets work, it's time to talk about how you use timezones in practice.
2. Time zone database
This is a picture of all of the different time zones in the world, as of 2017. They cut across countries, and within countries, and sometimes one is even totally surrounded by another one.
How could you possibly know all of these when you need to align your data to UTC? Do you need to look up the offset for each one in some big spreadsheet somewhere? Can't a computer help with this?
3. Time zone database
Thankfully, yes. There is a database called tz, updated 3-4 times a year as timezone rules change. This database is used by computer programs across many programming languages.
Because timezone information changes so quickly, it doesn't make sense to bundle it directly into Python. Instead, you will use a package called dateutil.
4. Time zone database
Let's start by making a timezone object that corresponds to the eastern United States, where our bicycle data comes from.
Within tz, time zones are defined first by the continent they are on, and then by the nearest major city.
For example, the time zone used on the eastern seaboard of the United States is 'America/New York'. We fetch this timezone by calling tz-dot-gettz(), and passing 'America/New York' as the string.
5. Time zone database
Here are a few more examples: 'America/Mexico_City'. 'Europe/London'. 'Africa/Accra'.
6. Time zone database
Let's look at our last ride again. Instead of specifying the UTC offset yourself, you pass the timezone you got from tz. Look at the result, and you can see that it's got the right UTC offset.
7. Time zone database
Even more excitingly, this same object will adjust the UTC offset depending on the date and time.
If we call datetime() with the time of our first ride, and pass in the same timezone info, we see that it gives us a different UTC offset.
We will discuss daylight savings time in the next lesson, but suffice to say that in some places the clocks change twice a year. Instead of having to look up when these things change, we just ask the timezone database to know for us.
tz includes rules for UTC offsets going all the way back to the late 1960s, and sometimes earlier. If you have data stretching over a long period of time, and you really care about getting the exact hours and minutes correct, you can use tz to put all of your date and timestamps on to a common scale.
8. Time zone database
Now that you have a basic understanding of using the tz class from dateutil, it's time to practice some examples!