Our application users have high expectations regarding timestamp accuracy, anticipating that our applications will intuitively recognize their time zones and adjust timestamps accordingly.
In this guide, we will explore various strategies for altering the time zone settings of the JVM. Additionally, we will delve into the challenges and common pitfalls encountered in managing time zones effectively.
By default, the JVM derives time zone data from the operating system. This data is then conveyed to the TimeZone class, responsible for maintaining the time zone and determining daylight saving time adjustments.
The method getDefault can be invoked to fetch the current operational time zone of the program. Additionally, one can retrieve a comprehensive list of time zones supported by the application through the use of TimeZone.getAvailableIDs().
Java utilizes the naming conventions established by the tz database for identifying time zones.
Changing the Time Zone
Setting an Environment Variable
Let’s begin by seeing how we can use an environment variable to change the time zone. We can add or modify an environment variable TZ.
For example, in Linux-based environments, we can use the export command:export TZ="America/Sao_Paulo"Copy
After setting the environment variable, we can see that the time zone of our running application is now America/Sao_Paulo:
Calendar calendar = Calendar.getInstance();
assertEquals(calendar.getTimeZone(), TimeZone.getTimeZone("America/Sao_Paulo"));Setting a JVM Argument
An alternative to setting an environment variable is setting the JVM argument user.timezone. This JVM argument takes precedence over the environment variable TZ.
For example, we can use the flag -D when we run our application:java -Duser.timezone="Asia/Kolkata" com.company.MainCopy
Likewise, we can also set the JVM argument from the application:System.setProperty("user.timezone", "Asia/Kolkata");Copy
We can now see that the time zone is Asia/Kolkata:Calendar calendar = Calendar.getInstance(); assertEquals(calendar.getTimeZone(), TimeZone.getTimeZone("Asia/Kolkata"));Copy
Setting the Time Zone From the Application
Finally, we can also modify the JVM time zone from the application using the TimeZone class. This approach takes precedence over both the environment variable and the JVM argument.
Setting the default time zone is easy:
Use this following line before you create the instance of Date:
TimeZone.setDefault(TimeZone.getTimeZone(“America/Los_Angeles”))
Global Settings
Note that each of the above approaches is setting the timezone globally for the entire application. In modern applications, though, setting the timezone is often more nuanced than that.
For example, we probably need to translate time into the end user’s timezone, and so a global timezone wouldn’t make much sense. If a global timezone isn’t needed, consider specifying the time zone directly on each date-time instance. Either ZonedDateTime or OffsetDateTime is a handy class for this.
Conclusion
In this guide, we explored multiple methods to adjust the time zone settings of the JVM. We discovered that it’s possible to alter the time zone through setting a system-wide environment variable, modifying a JVM argument, or making changes directly within our application through programmatic adjustments.