Aleksa Krstic
Aleksa Krstic
May 09, 2023
11 min read
May 09, 2023
11 min read

A comprehensive guide about time zones for software developers

A comprehensive guide about time zones for software developers

Ever wondered how software applications manage to tell the time accurately for users worldwide? The secret lies in mastering time zones within the software! As a developer, this skill is essential for ensuring your projects run smoothly, whether it's a travel app or a global collaboration tool.

In this all-in-one guide, we'll delve into the world of time zones, Universal Time Coordinated (UTC), and how they influence your software development. We'll cover the best techniques, clear up misconceptions, and offer practical examples to enhance your understanding of time zones. By the end of this guide, you'll be confident in tackling time-related challenges in your applications, ensuring a seamless experience for users across the globe.

Understanding time zones

A time zone, in simple terms, refers to a region or area on Earth where the local time is standardized and maintained uniformly for the sake of simplicity and coordination. Time zones are primarily based on the Earth's rotation around its axis, with each zone roughly representing one hour of the 24-hour day.

The concept of time zones was introduced in the late 19th century to address the confusion caused by differing local times, which arose as rail travel and communication technologies advanced. Before time zones, each city or town had its own local time, based on the position of the sun. This made scheduling and coordinating events across different locations challenging.

Time zones are generally centered around meridians of longitude, with each time zone spanning 15 degrees of longitude. The starting point for time zones is the Prime Meridian, which is 0 degrees longitude and runs through Greenwich, London, England. As you move eastward or westward from the Prime Meridian, time zones increase or decrease by one hour, respectively.

While time zones are theoretically based on strict longitudinal divisions, political boundaries and practical considerations often result in irregularly shaped zones. Some countries, such as China and India, have chosen to adopt a single time zone across their entire territory, despite spanning multiple theoretical time zones.

Here, you will find a recent time zone map that offers a visual representation of the numerous time zones covering our planet.

World time zones map

Standard Time vs. Daylight Saving Time

Standard Time and Daylight Saving Time are two different systems used worldwide to regulate timekeeping. Understanding their differences, as well as why they coexist, is essential for keeping track of time and adapting to changes in various regions.

Standard Time is the local time within a specific time zone that is based on the mean solar time of a particular longitudinal meridian. It is the default timekeeping system for most countries, ensuring consistency and simplicity in daily life. Standard Time does not undergo any adjustments throughout the year and is the basis for creating time zones worldwide.

In contrast, Daylight Saving Time (DST), or Summer Time as it's known in Europe, is a seasonal time adjustment practice that aims to extend the daylight hours in the evenings during the longer days of spring and summer. Typically, clocks are moved forward by one hour in the spring (usually referred to as "springing forward") and set back again by one hour in the fall ("falling back"). This practice is designed to make better use of daylight, potentially saving energy and allowing people to engage in outdoor activities for longer periods during the evening.

The use of Daylight Saving Time varies from country to country and sometimes even within a country. Many countries in North America and Europe, including the United States, Canada, and Germany, observe DST, while others like Japan, India, and China remain on Standard Time throughout the year. It is worth noting that the start and end dates of DST are not uniform across countries, and some countries may change their DST policies over time.

UTC: Universal Time Coordinated

Universal Time Coordinated (UTC) is the main time standard used around the world to keep time consistent across different places. It is the basis for everyday timekeeping and is widely used in computers, science, and communication. The main goal of UTC is to offer a single reference point for time synchronization, making sure timekeeping is accurate and the same everywhere.

In computer systems, UTC is very important for keeping timestamps accurate and consistent. It is usually the standard for saving date and time information, which allows computers to easily compare and sync time across different time zones. When showing local time to users, computers can add or subtract an offset to UTC based on the user's time zone. This ensures that the right local time is shown, no matter where the user is located.

UTC is often mentioned together with Greenwich Mean Time (GMT) and Zulu time. Although they are similar, there are small differences between these time standards. GMT is an older standard based on the average solar time at the Royal Observatory in Greenwich, London. UTC was created to account for small changes in Earth's rotation and is more accurate because it combines both solar time and atomic time. Zulu time is another name for UTC, mostly used in aviation and military contexts.

Handling time zones in computer systems

Dealing with time zones is crucial for keeping track of time accurately across different regions in computer systems. That's why computers use standardized time formats and methods, like UTC, to make it easier to convert and sync time data.

UTC serves as a consistent reference point, allowing computers to compare and sync time across numerous time zones. When displaying local time to users, computers apply an offset to UTC based on the user's location. For instance, if the UTC time is 12:00 PM, and a user is in New York (UTC-4), the computer shows the local time as 8:00 AM. Similarly, for a user in Paris (UTC+2), the local time displayed would be 2:00 PM. By using UTC, computer systems can operate with a single time zone, and the data shown to users is adapted according to their location and local time.

While you can store dates and times as strings, using timestamps is a more common and efficient way to handle time in computer systems. A timestamp is a unique number that represents a specific date and time, counting the seconds that have passed since the Unix epoch, which started on January 1, 1970, at 00:00:00 UTC.

Speaking of timestamps, it's worth mentioning the year 2038 problem. This potential issue could come up due to the limitations of 32-bit integer timestamps used in many computer systems. On January 19, 2038, these timestamps will exceed their maximum value, leading to incorrect date and time calculations. To fix this issue, developers are encouraged to switch to 64-bit timestamps, allowing computer systems to work properly for a much longer time.

Timezone example for developers

In this example, we'll use JavaScript to demonstrate how to manage dates. We'll show you how to convert a date into a timestamp, which calculates the milliseconds since midnight on January 1, 1970 (UTC), as well as how to format the date according to the user's language and location, like adjusting a UTC date for users in New York (UTC-4) and Paris (UTC+2).

Keep in mind that although we're using JavaScript here, other programming languages also offer ways to handle dates and times. To explore these features, refer to the documentation for each language.

const date = '2023-05-09T10:20:00.000Z';

const timestamp = new Date(date).getTime();
console.log(timestamp); // 1683627600000

const formattedNewYork = new Date(date).toLocaleString('en-US', { 
  timeZone: 'America/New_York' 
console.log(formattedNewYork); // 5/9/2023, 6:20:00 AM

const formattedParis = new Date(date).toLocaleString('fr-FR', { 
  timeZone: 'Europe/Paris' 
console.log(formattedParis); // 09/05/2023 12:20:00 

Common misconceptions about time zones

Time zones are an integral part of our daily lives and software development, but there are many misconceptions and myths surrounding them. In this text, we will dispel some common misunderstandings about time zones, GMT, UTC, Zulu time, and more.

  • GMT, UTC, and Zulu time: Many people believe that GMT, UTC, and Zulu time are the same, but there are subtle differences. GMT (Greenwich Mean Time), UTC, and Zulu are time standards that share the Prime Meridian as a reference point and don't observe daylight saving time. GMT, based on Earth's rotation, was the first global time standard, while UTC, its modern successor, combines Earth's rotation with atomic time for higher precision. Zulu, another name for UTC, is used primarily in aviation and military contexts. Although GMT is still informally used, UTC has become the preferred global timekeeping standard.

  • UTC offsets: A common misconception is that UTC offsets range only from -12 to +12 hours. In reality, there are also UTC +13 and UTC +14 time zones. These additional time zones, established to accommodate the unique geographic and political circumstances of certain regions, ensure that local times align more accurately with the natural day-night cycle. In some cases, these extended offsets are also employed to facilitate trade or communications with neighboring countries.

  • Non-integer offsets: Time zones are not always offset from UTC by whole hours. Some countries use half-hour or even quarter-hour offsets, like Iran (UTC +3:30), India (UTC +5:30), Nepal (UTC +5:45), and New Zealand (UTC +12:45).

  • DST variations: DST does not start or end at the same date and time for all countries.

  • DST in the Southern Hemisphere: DST does not follow the same pattern worldwide. In countries located in the Southern Hemisphere, summer occurs during the opposite months, causing DST to start and end at different times.

  • Partial state compliance with DST: In some cases, parts of a state may not follow the same DST rules as the rest of the state, such as the Navajo Nation in Arizona, USA.

  • Time zone changes: Countries may change their time zones, which are often dictated by politics and regulations rather than natural requirements. Regularly checking and updating your code is crucial for applications that rely on precise time tracking.

  • Time zone abbreviation confusion: Abbreviations like CST can represent multiple time zones (e.g., Central Standard Time and China Standard Time). Understanding the context surrounding the abbreviation is essential to avoid confusion.

  • Single time zone for large countries: Some countries, like China, use a single time zone across their entire territory, even though they span multiple longitudinal divisions.

  • Time zones for the poles: Time zones are rarely mentioned for the North and South Poles due to their inhospitable environments and lack of permanent human settlements.

Time zone best practices for software developers

Time zones play a crucial role in software development, particularly when creating applications for a global audience. To ensure accurate timekeeping, developers must follow best practices when working with dates and times. In this section, we'll discuss essential tips and strategies for handling time zones, daylight saving time changes, testing time-related code, generating UTC dates and times, localization of date and time formats, and more.

  • Store dates and times in UTC: Storing date and time values in UTC provides a consistent reference point, simplifying calculations, comparisons, and conversions between time zones. When displaying local time to users, apply the appropriate offset based on their location.

  • Generate UTC dates and times on the server-side: Whenever possible, generate UTC dates and times on the server-side to reduce errors and the risk of storing invalid data. However, in some cases, it may not be feasible to perform this on the server-side, and generating UTC on the client-side might be necessary. Be mindful of potential issues and validate user input thoroughly.

  • Choose the appropriate format for date and time storage: The performance of operations such as sorting and querying can vary depending on the format used for storing date and time values. Timestamps are a common format used in software systems, but be cautious when designing your system and consider potential issues like the year 2038 problem. Evaluate your requirements and choose a format that best suits your needs.

  • Convert time zones with care: Use reliable libraries or built-in functions provided by your programming language when converting between time zones. These tools account for time zone offsets, daylight saving time changes, and other factors, ensuring accurate conversions.

  • Handle daylight saving time changes: Be aware of daylight saving time (DST) changes and use reliable libraries or built-in functions to account for them when converting time zones or performing calculations.

  • Validate user input: Properly validate user input for dates and times to prevent errors and potential security issues. Consider using a standardized format, such as ISO 8601, to avoid ambiguity when accepting date and time values.

  • Localization of date and time formats: When converting UTC dates and times to locale-specific formats, consider the importance of localization to accommodate different date representations across regions and countries. For example, May 9, 2023, would be displayed as "05/09/2023" for a user in the USA and "09/05/2023" for a user in France. Using appropriate localization ensures a better user experience and prevents potential confusion.

  • Test time-related code thoroughly: Rigorous testing is essential to ensure the reliability of time-related code. Include tests for different time zones, DST changes, leap years, and edge cases to guarantee your application's smooth operation.


Managing time zones in software development is super important if you want to keep things running smoothly for users all around the world. By really digging into the nitty-gritty of time zones and UTC, you can confidently tackle time-related challenges and create a seamless experience for your users. Just remember to follow best practices like storing dates and times in UTC, using trustworthy libraries for time zone conversions, and testing thoroughly to make sure your software works perfectly across different time zones and adjusts for daylight saving time changes.

Speaking of user experience, localization is key to keeping users happy with your product. And as we've seen in this post, localizing dates and times is a crucial part of working with this kind of data. If you're working on localization and looking for a way to simplify your workflow, try Localizely for free. It might just be the tool you need to make your life a whole lot easier!

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Aleksa Krstic
Aleksa Krstic

Aleksa is a Software Engineer at Localizely. Over the past few years, Aleksa has been working in the field of software localization. In his free time, he enjoys playing guitar and writing tech posts.

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