1. Starting Daylight Saving Time
Some places change their clocks twice a year to create longer summer evenings. This practice is called daylight saving time, but it would better be called daylight shifting time. In some countries it is called "summer time".
Dealing with daylight saving time can be one of the most fiendish challenges in dealing with dates and times.
To keep things simple, let's start with the situation where the clocks move forward in the spring. In the next lesson, we'll discuss handling the opposite case, when the clocks move back in the fall.
2. Start of Daylight Saving Time
Let's look at an example.
On March 12, 2017, in Washington, DC, the clock jumped straight from 1:59 am to 3 am. The clock "springs forward". It never officially struck 2 am anywhere on the East Coast of the United States that day.
3. Start of Daylight Saving Time
Just like before, to make our clock in Washington, DC comparable to clocks in other places, we need to represent it with a UTC offset. Only now the UTC offset is going to change.
On this date, at 1 AM in Washington, DC, we were in Eastern Standard Time. It was 6 AM UTC, a five-hour difference. At 3 AM in Washington, DC, we were in Eastern Daylight Time. It was 7 AM UTC, a four-hour difference.
4. Start of Daylight Saving Time
Let's see the same thing in code. To be as clear as possible, let's create the UTC offsets by hand for now instead of using dateutil-dot-tz.
We start by creating a datetime object, spring_ahead_159am, for March 12th, at 1:59:59, without any timezone information. We print the results out with isoformat() to check that we have the time right, and we make another object for spring_ahead_3am.
We subtract the two datetime objects and ask how much time has elapsed by calling total_seconds(). As expected, they're an hour and one second apart.
5. Start of Daylight Saving Time
As before, to fix problems with comparing datetimes we start by creating timezone objects. We define Eastern Standard Time, or EST, using the timezone constructor. We set the offset to -5 hours.
Similarly, we define Eastern Daylight Time, or EDT, with an offset of -4 hours.
6. Start of Daylight Saving Time
We assign our first timestamp, at 1:59 am to be in EST. When we call isoformat(), we see it has the correct offset. We assign our second timestamp, at 3:00 am, to be in EDT, and again check the output with isoformat().
When we subtract the two datetime objects, we see correctly that one second has elapsed. Putting things in terms of UTC once again allowed us to make proper comparisons.
7. Start of Daylight Saving Time
But how do we know when the cutoff is without looking it up ourselves? dateutil to the rescue again. Just like before when it saved us from having to define timezones by hand, dateutil saves us from having to know daylight savings rules.
We create a timezone object by calling tz-dot-gettz() and pass our timezone description string. Recall that since Washington, DC is in the America/New_York time zone, that's what we use.
Once again we create a datetime corresponding to 1:59 am on the day that the east coast of the US springs forward. This time though, we set the tzinfo to eastern time. Similarly, we create a datetime set to 3 am on March 12th, and when we set tzinfo to be eastern time, dateutil figures out for us that it should be in EDT.
8. Daylight Saving
In this lesson, we covered "spring ahead". Let's try some examples of working with datetimes that handle a switch into daylight saving time.