Time zones

1. ZonedDateTime class

There is another very interesting class in the Date Time API: the ZonedDateTime class. Its main purpose is to make it convenient to work with dates in different time zones.

LocalDate is great for representing dates. For example, birthdays. My birthday is March 15th no matter where I am. This is an example of a date.

LocalTime is great to describe time, like the time set on an alarm clock: I set the alarm for 5:00 am and it doesn't matter where I am. 5:00 am is 5:00 am. This is an example of working with time.

Now let's say that we're writing an application that books flights. Planes take off and arrive based on the local time. The plane is in the air for a fixed time, but the time zones can change.

Time zones

By the way, time zones are a real mess. And if you think that there are 24 time zones, you are greatly mistaken.

For example, the time in India differs from Greenwich Mean Time by five and a half hours: GMT+5:30. Some countries switch to daylight saving time, and others do not. What's more, different countries switch to summer time at different times of the year.

And some countries pass laws that cancel daylight saving time, or reintroduce it, or cancel it again.

In any event, the world has time zones, and within each time zone there is one time. Time in different zones may coincide during certain periods of the year, and then differ during other periods. Time zones are usually named after major cities located in them: Europe/Monaco, Asia/Singapore, but there are also exceptions — US/Pacific.

Officially, there are 599 time zones at the moment. Think about it: 599. That's far from 24. Welcome to the global world.

The ZoneId class from the java.time package is used to store a time zone in Java.

By the way, it has a static getAvailableZoneIds() method, which returns the set of all currently known time zones. To get a list of all zones, you need to write the following code:

for (String s: ZoneId.getAvailableZoneIds())
   System.out.println(s);

Asia/Aden
America/Cuiaba
Etc/GMT+9
Etc/GMT+8

To get a ZoneId object by its name, you need to use the static of() method;

ZoneId zone = ZoneId.of("Africa/Cairo");

Cairo

2. Creating a ZonedDateTime object

When creating a ZonedDateTime object, you need to call the class's static now() method and pass a ZoneId object to it.

ZoneId zone = ZoneId.of("Africa/Cairo");
ZonedDateTime time = ZonedDateTime.now(zone);
System.out.println(time);

2019-02-22T11:37:58.074816+02:00[Africa/Cairo]

If you don't pass a ZoneId object to the now() method (and that is allowed), then the time zone is determined automatically based on the settings of the computer that is running the program.

Example:

ZonedDateTime time = ZonedDateTime.now();
System.out.println(time);

2019-02-22T13:39:05.70842+02:00[Europe/Helsinki]

Converting a global date to a local one

One of the interesting features of ZonedDateTime is its ability to convert to a local date and time. Example:

ZoneId zone = ZoneId.of("Africa/Cairo");
ZonedDateTime cairoTime = ZonedDateTime.now(zone);

LocalDate localDate = cairoTime.toLocalDate();
LocalTime localTime = cairoTime.toLocalTime();
LocalDateTime localDateTime = cairoTime.toLocalDateTime();

3. Working with time

Like the LocalDateTime class, the ZonedDateTime class has lots of ways to get individual elements of a date and time. Here is a list of these methods:

int getYear()

Month getMonth()

int getMonthValue()

int getDayOfMonth()

DayOfWeek getDayOfWeek()

int getDayOfYear()

int getHour()

int getMinute()

int getSecond()

int getNano()

All the methods are perfectly analogous to the methods of the LocalDateTime class. And, of course, the ZonedDateTime class has methods that let you work with dates and times. That said, the object on which the methods are called does not change. Instead, they return a new ZonedDateTime object:

plusYears(int)

plusMonths(int)

plusDays(int)

plusHours(int)

plusMinutes(int)

plusSeconds(int)

plusNanos(int)

minusYears(int)

minusMonths(int)

minusDays(int)

minusHours(int)

minusMinutes(int)

minusSeconds(int)

minusNanos(int)

We won't provide any examples, since we think everything here is clear by analogy with the classes we just considered.